RESEARCH ARTICLE www.small-journal.com Nanostructure Tuning of Gold Nanoparticles Films via Click Sintering Massimo Urban, Giulio Rosati,* Gabriel Maroli, Flavio Della Pelle, Andrea Bonini, Laszlo Sajti, Mariangela Fedel, and Arben Merkoçi* 1. Introduction Colloidal metal nanoparticles dispersions are commonly used to create functional printed electronic devices and they typically require time-, energy- Colloidal gold has been used for health and equipment-consuming post-treatments to improve their electrical and treatments and art for thousands of years. In 1875, Michael Faraday found a way mechanical properties. Traditional methods, e.g. thermal, UV/IR, and to produce it synthetically,[1] and since microwave treatments, limit the substrate options and may require expensive then, photonic, catalytic, and other prop- equipment, not available in all the laboratories. Moreover, these processes erties of gold in its nanometric structure also cause the collapse of the film (nano)pores and interstices, limiting or have been studied and used in numerous impeding its nanostructuration. Finding a simple approach to obtain complex fields.[2–6] The versatility of this material has been exploited creating nanofunctional sur- nanostructured materials with minimal post-treatments remains a challenge. faces by inkjet printing taking advantage In this study, a new sintering method for gold nanoparticle inks that called as of the colloidal stability of gold nanoparti- “click sintering” has been reported. The method uses a catalytic reaction to cles (AuNPs) dispersions. In the last few enhance and tune the nanostructuration of the film while sintering the years, nanostructured materials have been metallic nanoparticles, without requiring any cumbersome post-treatment. commonly integrated in biomedical de- This results in a conductive and electroactive nanoporous thin film, whose vices, demonstrating tremendous improve- ments in their analytical performance.[7–9] properties can be tuned by the conditions of the reaction, i.e., concentration of Nanostructured electrodes have thus gar- the reagent and time. Therefore, this study presents a novel and innovative nered a lot of attention for their poten- one-step approach to simultaneously sinter gold nanoparticles films and tial as effective transducing elements for create functional nanostructures, directly and easily, introducing a new the detection of a wide range of biomark- concept of real-time treatment with possible applications in the fields of ers, but also for catalysis and energy stor- age among other applications.[10–12] There flexible electronics, biosensing, energy, and catalysis. are several approaches for the electro- chemical performance enhancement via M. Urban, G. Rosati, G. Maroli, F. D. Pelle, A. Bonini, A. Merkoçi F. D. Pelle Nanobioelectronics and Biosensors Group Department of Bioscience and Technology for Food, Agriculture, and Catalan Institute of Nanoscience and Nanotechnology (ICN2) Environment Campus UAB, Bellaterra, Barcelona 08193, Spain University of Teramo E-mail: giulio.rosati@icn2.cat; arben.merkoci@icn2.cat Campus “Aurelio Saliceti” via R. Balzarini 1, Teramo 64100, Italy M. Urban A. Bonini Doctorado en Biotecnología Department of Chemistry and Industrial Chemistry Universitat Autònoma de Barcelona University of Pisa Campus de la UAB, Bellaterra, Barcelona 08193, Spain via Giuseppe Moruzzi 13, Pisa 56124, Italy L. Sajti, M. Fedel Nano-Engineering Group The ORCID identification number(s) for the author(s) of this article RHP Technology GmbH can be found under https://doi.org/10.1002/smll.202306167 Seibersdorf 2444, Austria © 2023 The Authors. Small published by Wiley-VCH GmbH. This is an A. Merkoçi open access article under the terms of the Creative Commons Catalan Institution for Research and Advanced Studies (ICREA) Attribution-NonCommercial-NoDerivs License, which permits use and Passeig de Lluís Companys, 23, Barcelona 08010, Spain distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. DOI: 10.1002/smll.202306167 Small 2023, 2306167 2306167 (1 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 1. Overview of the fabrication of the devices, from inkjet printing to click sintering. a) Scheme of generic printing and sintering protocol for AuNPs inkjet-printed films; b) illustrative representation of the sintering mechanism involving the reaction with the sintering agent (NaBH4 ) and the local nanoenviroment at the interphase. nanostructuration, including the decoration of the electrodes’ ment are required. In this direction, chemical approaches have surface with NPs, electroplating, or the fabrication of the en- also been explored to induce sintering of printed nanomaterials, tire transducer with metal nanoparticles (MNPs).[3,6,13–15] In al- including gases, ionic compounds, and salts, sometimes using ternative to MNPs depositions, other classical approach to create cumbersome procedures with difficult (or impossible) industrial homogeneous nanoporous gold network include, chemically or implementation or losing the inherent nanostructure properties electrochemical dealloying, template-assisted depositions, etch- of the nanoparticles.[50–55] In this context, chemically induced sin- ing, and chemical depositions.[14,16,25–28,17–24] tering of noble MNPs inks remains an important challenge. Despite several options available, there are few methods allow- In this study, we propose a straightforward method called ing the low-cost, efficient, and easy fabrication of nanostructured “click” sintering for tuning the nanostructure of AuNPs conduc- metal-based conductive surfaces, and even less on flexible or de- tive films, compliant with paper and polymeric substrates. The formable plastics and paper substrates. method consists in an instantaneous reaction at the AuNPs par- In the last decade, inkjet printing using MNPs inks has be- ticle interphase, of a solution of sodium borohydride (NaBH4 ) at come a popular fabrication technique in the fields of flexible elec- room temperature (Figure 1b). We have observed that the NaBH4 tronics and biosensing.[29–32] Compared to other printing tech- treatment of AuNPs films modulates the nanostructuration, giv- niques, inkjet printing offers higher resolution, reduced use of ing improved electrochemical features and making possible the solvents, and freedom and flexibility in terms of layouts. After easy fabrication of ad-hoc electrodes/surfaces for various appli- deposition, the printed films typically undergo a process called cations, such as electrochemical biosensors, Surface Enhanced sintering. Sintering is a thermodynamically induced growth Raman Scattering (SERS) substrates, gas sensors, flexible elec- process that increases the dimensions of MNPs, leading to a tronics, and wearable devices.[56] The proposed method does not smaller surface-to-volume ratio, less surface chemical potential, require any equipment and can be performed virtually in any lab and higher overall stability.[33] The process leads to conductive in just a few minutes. Furthermore, we demonstrated that the paths in MNPs-based films and starts with the curing of the de- process can be automated via integration into an inkjet printer. posited film, starting from the removal of the solvent and reach- ing the removal of the capping agent, which hinders the elec- 2. Results and Discussion trical/electrochemical response.[34–36] Immediately after, the real sintering process starts, forming junctions between the MNPs In this work, we have shown that the concentration of NaBH4 creating conductive paths, and improving the film’s mechanical modulates the nanostructuration of AuNPs films, improving strength.[37–39] Generally, this causes the collapse of the porous electrochemical properties, and giving them a peculiar optical structure of the nanoparticle interphase and the consequent loss activity, as will be described in the next paragraphs. This click- of the film nanostructure. sintering method takes advantage of the nanochemical phenom- The selection of the appropriate sintering process for inkjet- ena occurring on the surface of the NPs, i.e., a catalytic exother- printed films is based on the type of ink, the printing sub- mic reaction inducing the formation of a nanoporous surface of strate, and on eventual time/scalability-related requirements. metal NPs. NaBH4 has already proved to be effective in removing For example, in mass fabrication lines, a photonic approach thiols, polyvinylpyrrolidone (PVP), and other ligands from col- is usually preferred, but the most typical way is via thermal loidal gold and other metal NPs,[57,58] however, to the best of our annealing.[38,40,49,41–48] Flexible electronics and biosensing appli- knowledge there is no report about its use as nanostructuring cations are often based on temperature-sensitive substrates such sintering agent for AuNPs-based inks in printed electronics and as plastics or paper; hence, alternative strategies to thermal treat- nanobiosensing. Small 2023, 2306167 2306167 (2 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 2. Electrical characterization of the inkjet-printed gold films produced via dipping (a) and via inkjet sintering (IJS) (b). a) Sheet resistance values for the AuNPs-ink printed squares (in red) and resistance values of the printed lines (in black), after the 10 min treatment with different concentrations of NaBH4 ; the squares represent the calculated resistance using the sheet resistance and the aspect ratio of the trace. The top right insets in the two plots show the layouts of the respective printed structures. b) Sheet resistance values for the 20 mm squares, after real-time inkjet sintering on Kapton® at increasing numbers of layers, it is showed a threshold in the relative amount sintering solution (NaBH4 0.5 m ink). The columns represent average values and the bars the respective standard deviations (n = 3). Herein, AuNPs-films conductivity, nanostructure morphology, excellent correlation between the two measurements (see SI for elemental composition, and the material’s surface chemistry the calculation). We used the average thickness of the film printed were investigated after this simple click sintering treatment to over Mitsubishi substrate (Figure S2, Supporting Information), shed light on its mechanism and to define the properties ac- combined with the measured sheet resistance, to calculate the quired by the film. Additionally, the film’s electrochemical fea- conductivity and resistivity. A concentration of NaBH4 1 m proved tures have been studied using two different redox molecules (e.g., to be the best value in terms of resulting electrical performance, methylene blue and ferro/ferricyanide) to pave the way for po- with a conductivity of 5.14*107 S m−1 , equivalent to 1.25% of the tential applications in flexible electronics and electrochemical bulk conductivity of gold (4.11*107 S/m). Although the value is nanobiosensing. not particularly high, compared with the state-of-the-art AuNPs inks and printed films, this places our method somewhere in the middle in term of performances, but with a clear advantage in 2.1. Electrical Characteristics of the Nanostructured Films terms of time, temperature of the process and with the result of an extended nanostructuration (Tables S1 and S3, Supporting In- To prove the effectiveness of the sintering process we evaluated formation). changes in the film sheet resistance, resistivity, and conductiv- While performing the alternative sintering method, i.e., using ity. Two-wire (2P) and four-point probe (4P) methods were used inkjet printing, the sheet resistance remains high and not repro- to measure both the resistance of printed lines and the sheet ducible, indicating a heterogeneous treatment, up to two NaBH4 resistance of the film. Simple lines (20 mm long and 600 μm printed layers (Figure 2b). With three layers or more, a sharp de- wide) and 20 mm side squares were printed on a commercially crease in resistivity has been evidenced with a higher degree of coated PET substrate via inkjet printing with a consumer printer. reproducibility. Given the small volume deposited with the IJP The inkjet-printed films appeared gold-colored after printing but process, multiple layers are therefore necessary for an effective resulted non-conductive, as expected in absence of a sintering sintering of the films. We found that three layers of sintering treatment. The AuNPs-films were then treated by immersion in agent per layer of AuNPs ink produced the best result. Notewor- 0.05 m NaOH solutions containing different concentrations of thy, the resistivity obtained with IJS turns out to be similar to the NaBH4 for 10 min, ranging from 1 mm to 1 m. The films were one obtained via dipping. then rinsed with MilliQ water and dried with nitrogen. Nanostructure. The colors and morphology of the printed The sheet resistances of the squares and the resistances of the films after the click sintering with different NaBH4 concentra- lines have been measured for each concentration of NaBH4 . A tions are reported in Figure 3. From a macroscopic point of view, decrease in the resistance was observed with increasing concen- the treatment produced a gradual change in the color of the gold trations of NaBH4 for both the tested printed layouts (Figure 2a). film, from gold to deep brown, proportionally with the NaBH4 The pristine untreated films did not result conductive; thus, they concentration (Figures 3a,e,i,m,q,u). do not appear in the plots. Figure 2b shows the resistivity ob- The SEM micrographs demonstrate that the nanostructure tained for inkjet sintering (IJS), performed by placing the NaBH4 significantly varies with different concentrations of the sinter- solution (0.5 M) in a dedicated cartridge of the printer, and per- ing agent. The untreated AuNPs film shows a uniform distri- forming different passages (named layers) onto the inkjet printed bution of spherical NPs on a dense underlying layer, whereas AuNPs-film on Kapton substrate (Figure S1, Supporting Infor- a dense 3D-network with cavities and pores extending over mation). the whole nanostructure is observed with NaBH4 concentra- In Figure 2a, a decrease of the (sheet) resistance has been ev- tions over 0.1 m (Figure 3g–l,m–r). This effect could be linked idenced with concentrations lower than 0.1 m NaBH4 , with an to the necking process that occurs during the sintering after Small 2023, 2306167 2306167 (3 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 3. The chemical sintering at different concentrations tunes the proprieties of the gold films from the micro to the nanometric scale. First column reports the optical pictures of the films treated with different concentrations of NaBH4 , showing the color variations from yellow to dark brown. Second, third, and fourth columns, SEM images at increasing magnifications of the sintered AuNPs-film treated with different concentrations of NaBH4 . Scale bar dimension from left to the right 1 mm, 1 μm, and 200 and 50 nm. the capping layer removal. This process creates junctions be- NaBH4 is a promising reagent for hydrogen production in both tween NPs, making the film conductive (they “click”), and in- solid and liquid phases, and its dissociation in water has been creases the porosity and nanostructure of the film, resulting well-studied in the literature.[59–61] Alkaline pH stabilizes NaBH4 , in a higher surface-to-volume ratio. TEM micrographs confirm by reversing the equilibrium of the hydrolysis in water.[62,63] No- the formation of the neck between the particles (Figure S2, ble metal catalysts have been used to dissociate NaBH4 in an Supporting Information). This process can be described as a exothermic reaction, which produces hydrogen gas.[60,64,65] The catalytic-induced sintering of NPs, which occurs without the need proposed mechanism involves intermediate species where for- for external sources of energy. The intrinsic chemical energy mal hydride is bonded to the surface of the particle and then re- of the reaction is harnessed, confined to the nanodomains of acts with water in the rate-determining step.[60] When gold films the solid-state film triggering the sintering and forming a web- are exposed to the sintering solution, a visible color change ac- like structure. To our knowledge, this is the first report about companies the production of hydrogen gas that is especially in- catalytically induced sintering as a tunable nanostructuration tense at higher concentrations of NaBH4 (Figure S3b and Video mechanism. S2, Supporting Infromation). SEM images reveal the presence Small 2023, 2306167 2306167 (4 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 4. Time-concentration SEM study of the morphological effects of the NaBH4 via dip sintering. Changes in morphology have been monitored at different concentrations for times spanning from 1 to 60 min. As in the previous figure, 0.1 m appears as the threshold concentration NaBH4 for triggering an extended nanostructure change. Conversely, time does not appear to have an evident effect on the morphology, as it has been observed also during the inkjet sintering (see Figure 2), which is a real-time process. Scale bar 200 nm. of cavities and pores in the printed structure that can only be stops when thermodynamic equilibrium is reached, i.e., when explained by the micro-explosions of hydrogen gas produced in desorption/reabsorption of PVP reaches balance and all the ac- the deeper regions of the film (Figure S4, Supporting Informa- tive crystalline planes of the smaller particles have reacted. tion). The rate of the catalytic reaction, and therefore hydrogen The coalescence modes of the AuNPs change according to the production, is heavily affected by the concentration of NaBH4 . NaBH4 concentration, with two main driving forces, i.e., i) de- At lower concentrations, the NPs undergo competition between capping rate and ii) spatial confinement. We think that the most hydrogen production and energy production via exothermic reac- probable way for it is that the hydrogen produced during the re- tions, leading to their coalescence. The sintering process occurs action interacts with the PVP via destabilization and mechani- rapidly, and the morphology doesn’t change significantly within cal removal, most probably following a similar reaction occur- 1 h (Figure 4). It is widely accepted that there is a relationship ring during the electrochemical cleaning of the gold electrodes between the activity of the catalyst and particle size, with smaller via oxidations and subsequent reductions.[67] With higher con- particles being increasingly more active compared to the bigger- centrations of sodium borohydride (≥0.1 m) the PVP destabiliza- sized ones.[66] We speculate that the smaller particles, which are tion rate is higher, and the removal of the capping agent results nested between the larger ones, are the main actors in the reac- almost instantaneous, or in the range of seconds (Figure 4). The tion. This is suggested by the “fishing net” structure that forms arrangement of the AuNPs in the film is ruled by the printing, the with bigger particles connected to each other and separated by drying process, and the substrate surface properties, forcing the smaller NPs and narrow necks. The network-forming process NPs into defined positions. During the sintering the NPs start Small 2023, 2306167 2306167 (5 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 5. Physiochemical characterization of the printed AuNPs films at the different concentrations of the sintering agent. a) EDX maps of the pristine and of the 1 m NaBH4 treated devices with the compositional maps showing the distribution of the main elements found (Au, O, Na, and C). The overall nanostructure rearrangement with the increased porosity is evident in the overall structure. However, the single elements do not show relevant alterations. b) The atomic concentrations of the elements (Au, C, O, N, Na, Cl, and Mg) evidence of an increase in the gold content and a decrease of the carbon one with increasing NaBH4 concentrations. No contaminants were evidenced on the surface of the treated films. c) The XPS measurements of the gold 4f5/2 and 4f7/2 peaks showed a constant separation, and d) the intensities do not appear to change proportionally with the NaBH4 concentration. Scale bar 500 nm. necking in all directions, toward the closest NPs, creating a 3D gen, and sodium) appears uniform and homogenous. The XPS porous network, with a larger degree of freedom close to the shows that the surface concentrations of carbon and nitrogen are surface. Analyzing the bottom layers of the film, a more com- decreasing with increasing concentrations of NaBH4 , as an ef- pact structure can be appreciated (Figure S5, Supporting Infor- fect of the PVP capping agent removal. It is to be noted that no mation). The situation is similar in the case of inkjet sintering boron is present on the surface of the film, contrary to what was (IJS) where the nanostructure changes after reaching a thresh- expected using an excess of reagent. Metaborate products pro- old number of layers, i.e., 3 (Figure S6, Supporting Information). duced during the reaction are soluble in water and negatively For low concentrations (or jetted volumes for the IJS) of sodium charged, as the NPs used in this work (𝜁 -potential −40 mV in borohydride, the destabilization of the PVP capping layer results water see Table S5, Supporting Information). The electrostatic slower, with no apparent change on the film surface, despite mak- repulsion explains the absence of the boron species and the ap- ing it conductive. The discrete nature of the printing process, de- pearance of Na, which is positively charged and tends to accumu- positing the sintering agent layer-by-layer, allows for selectively late on the surface, increasing with the concentration of NaBH4 . tuning the structure by using an appropriate number of layers. The high-resolution XPS spectra of the Au 4f show the doublets corresponding to the photoemission peaks of the 4f5/2 and 4f7/2 orbitals, at binding energies of 87.79 and 84.12 eV, respectively, 2.2. Composition and Chemical State with a peak-to-peak distance of 3.67 eV. The result is coherent with the NPs being in the metallic form with formal oxidation Energy-dispersive X-ray Spectroscopy (EDX) and X-ray Photo- state Au0 .[68] Both the distance and the position of the peaks re- electron Spectroscopy (XPS) analysis allowed the identification main constant with the treatment. The peak intensities change of the main components and chemical state of the gold film sur- accordingly to the extent of the removal when the concentration faces, their distribution, and their percentage variations in re- of NaBH4 is increased. The absence of variation in the gold peak sponse to the treatment with NaBH4 (Figure 5). suggests that no relevant interaction between the gold and the The different elements present after the treatment of the film sodium borohydride takes place, with no change in the formal are reported on the elemental maps (Figure 5a,b) and the XPS oxidative state of the gold due to the treatment. The examination survey (Figure S7, Supporting Information). The distribution of of the deconvolution of the carbon peaks provides insights into the analyzed elements in the sampled area (gold, carbon, oxy- possible reactions or interactions with the polymer. It has been Small 2023, 2306167 2306167 (6 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com reported that PVP has different binding modes on metal NPs, 2.3. Electrochemical Properties depending on the size, length of the chain, and branching.[69,70] Given the broad size distribution of the AuNPs used in this work Cyclic voltammetry (CV) measurements in presence of (polydispersity index>0.1) it is reasonable to expect a contribu- ferro/ferricyanide as redox probe, have been performed to tion coming from the different binding modes of the polymer. evaluate the electron transfer ability of the AuNPs electrodes For other metals, the interaction with the charge transfer with the treated with different concentrations of NaBH4 . A conductive polymer translates into additional components in the XPS peak film with poor electroactivity has been formed with the lowest model as formal species with charge +1, related to the chemisorp- NaBH4 concentration, but already with 0.01 m NaBH4 , the tion of the oxygen onto the metal surface.[71–73] However, this is electrochemical response showed a significant improvement, not what was observed (Figure S8, Supporting Information). The corresponding to the transition behavior between macroelec- ratio between the components related to the Au (0) and the Au trode and nanostructured electrode (Figure 6). (I), does not change during the different treatments, and remains Figure 6a clearly shows that the peak intensity increases, constant even after changes in concentration of several orders of and the peak-to-peak separation decreases proportionally to the magnitude. The carbon peaks also remained constant and did NaBH4 concentration used for the treatment (Figure 6a,b). Fur- not change significantly after the treatments at different concen- thermore, for all the electrodes, the peak currents varied linearly trations, further confirming that NaBH4 interacts directly only with the square root of the scan rate and with an anodic and with the particles and with no other species (Figure S9, Support- cathodic peaks ratio corresponding to 1, in accordance with the ing Information). A closer inspection reveals that the peak of the Randles-Sevcik equation, showing the characteristics of a quasi- amidic carbon progressively shifted toward lower binding ener- reversible reaction.[82] The electron transfer constant k0 , a value gies at lower concentrations of NaBH4 but returned to its normal defining the efficiency of the redox process, increases linearly position at higher concentrations, suggesting an increase in the with the concentration of NaBH4 due to the exposition of more re- surface of the NPs where the PVP has been displaced but not dox active centers with the treatment (Table S2, Supporting Infor- bonded. It is possible to propose different interpretations of this mation). For the higher NaBH4 concentrations, the peak currents phenomena: i) according to Ansar et al,[57] we found that the sub- in the CVs increase linearly with the scan rate, up to≈500 mV s−1 stitution of the PVP with hydride does not affect the gold oxida- after which a small secondary shoulder at high potentials (̴ 0.45 V) tion state, and no change in the electronic shell of the Au can be appears. This shoulder peak could be attributed as a feature of detected; ii) the NaBH4 interacts directly with the NPs, producing nanoporous gold electrodes, due to the complex kinetics and dif- hydrogen and mechanically detaching the coordinated PVP, as it fusion typical for porous systems.[83] has been demonstrated in the mechanism related to the electro- We then performed CVs scans in 0.1 m sulfuric acid, and cal- chemical cleaning with sulfuric acid.[67,74] The higher concentra- culate the total electroactive surface area (ECSA) of the AuNPs, tions of the sintering agent also modulate the nanostructuration, by extracting the area corresponding to the gold oxide reduc- with distinctive electrochemical and optical proprieties, such as tion peak (≈0.7 V), knowing the theoretical consumed charge the color change caused by the change in the scattering proper- for polycrystalline gold (410 μC cm−2 ) (Figure S13, Supporting ties of the material due to the newly-formed nanostructure.[23,75] Information).[67] This value can be normalized to the geometri- The morphological change of the surface with the treatment cal area, the roughness factor (Rf ), calculated as the ratio between could explain the color variation of the films , [76,77] because of the effective area Aeff and the geometrical area Ageo (Figure 6d; its different light absorbance. These data prove that the color Table S2, Supporting Information). changes of the films are not imputable to chemical changes of The Rf tops to a remarkable value of 40, comparable or even the film but are due to changes in the nanostructure. SERS higher than some of the commonly used method for producing measurements also offer a view of the effects of the nanos- nanoporous gold, with the advantages and unique features of tructuration. Rhodamine was used as a probe, and the inten- speed, temperature, and integration provided by our treatment sity of its characteristic peak (1626 cm−1 ) was taken as a refer- (Table 1). The value decreases at the threshold NaBH4 concentra- ence signal.[78,79] The intensity of the Rhodamine 6G peak (1626 tion of 0.1 m, where starts increasing again (Figure 6). This phe- cm−1 ) decreases linearly increasing the concentration of NaBH4 nomena is correlated to the transition in morphology that hap- after 0.01 m (Figure S10, Supporting Information). More nanos- pens at that specific concentration and clearly visible from the tructured films showed higher porosity and irregular shapes SEM analysis (Figure 3). The nanoparticles “click” together losing (Figure 3). On average the distance and gaps between the NPs their sphere-like shape due to the sintering. This causes a loss of are larger, which implies fewer hot spots enhancing the Raman total surface area, because some of the crystalline planes available scattering. Numerous factors influence the SERS phenomena, for the redox process coalesce forming new entanglements. It is such as the nature and the morphology of the metal on the sub- also important to compare this value with the electrode transfer strate; among others, particularly significant is the assembling constant, k0 , calculated before. Even if with higher surface area, and the NPs shape.[79–81] In this case higher concentrations of using lower NaBH4 concentration, causes inefficient de-capping sintering agents create more disorganized and porous structures of the polymer layer. This causes the films to be higher in total with active centers distancing each other. This also opens the way surface area, but overall less sensitive to the redox probe (Table to possible strategies of tuning the optical proprieties of the film S2, Supporting Information), compared with the films treated changing the concentration, and possibly having a film that is with higher concentrations. Also, the differences in the areas cal- conductive and at the same time optically active toward the SERS culated with the Randles-Sevick equation and the integration of detection mechanism. the gold oxide peak in the CV in sulfuric acid, suggest us that not Small 2023, 2306167 2306167 (7 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH Table 1. Comparison table of the main protocols for the fabrication of nanoporous gold. Ref. Protocol Substrate Tmax /Tnano a ) [°C] tb ) [h] Rf c) Pore size [nm] Reagents Comments [16] Dealloying Gold 120 2-3 540 100 ZnCl2 in benzyl alcohol Requires alloying wire Small 2023, 2306167 [17] Electrochemical Gold-disk RTd) <0.01 35 N.S.e) HCl 2m None etching electrode [14] Chemical deposition MCEf) 60 2 7.3 N.S. Au3+ solution in NH2 OH Multiple cycles for total membrane coverage of the paper [18] Drop casting Gold RT <10 300 N.S. For AuNPs synthesis Multiple casting is necessary www.advancedsciencenews.com electrode [19] Template-assisted COPg) 160/RT 2 N.S. 120 Many (for fabrication of the Metal is sputtered on the deposition resin mold) nanoimprinted mold [20] Photonic PETh) 120/RT 20 N.S. 44-48 Many (for polymer Multiple steps for the synthesis) polymer synthesis, sintering assembly and removal. [21] Hydrogen-bubble GCEi) RT 0.02 135 30-50 H2 SO4 and Electrodeposition method AuCl4 − [22] Net shaping Cu wire 600/RT 96 25–45 HNO3 , Cu and Au solution Requires alloying step at HT. Electroplating/corrosion method tamplate cycles. [23] Electrochemical Si wafer RT 0.3 32 N.S. HF and DMF Sputtering on chromium 2306167 (8 of 13) etching [24] Electro- C j) RT 0.1 14 N.S. PB k) , AA l) solution Anodization followed by reduction of the gold deposition oxide layer [25] Gold leafs Kapton tape RT 0.05 36.3 5-25 HNO3 Deposition of the leaf, and selective etching via polymer mask. [26] Electrochemical Au RT 0.1 8.5 N.S. H2 SO4 Electrochemical etching of the gold electrode [27] Electrochemical+ Gold 600/RT 18 232 N.S. HNO3 Requires alloying, electrodeposition followed Dealloying wire by dealloying [28] Metal-induced GCE 110/RT 0.5 ≈3 N.S. H2 O2 Deposition GCE via sputtering and chemical crystallization etching This work Click PET, Kapton RT <0.5 ≈40 7-15** NaBH4 Deposition and sintering can be integrated in a sintering consumer inkjet printer a) b) Tmax /Tnano = maximum temperature of the process/ temperature for the step inducing the porosity/; t = time (time is calculated by considering the total time from the preparation of the sample to the step of nanos- c) d) e) f) g) h) i) www.small-journal.com © 2023 The Authors. Small published by Wiley-VCH GmbH tructuration; Rf = roughness factor (Aeff /Ageo ); RT = room temperature; N.S. = not specified; MCE = mixed cellulose ester; COP = cyclo-olefin polymer; PET = polyethylene tereftalate; GCE = glassy carbon j) k) l) ∗∗ electrode; CD = compact disk; PB = phosphate buffer; AA = ascorbic acid; = see Supplementary Figure S16 (Supporting Information) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 6. Electrochemical characterization of the treated electrodes. a) Cyclic voltammetry measurements performed on 4 mm square AuNPs printed working electrodes with external Ag/AgCl reference and Pt counter electrodes in 5 mm ferro/ferricyanide in 10 mM phosphate saline buffer (PBS, pH 7.4, 23 °C). b) Anodic peak currents recorded versus the scan rate for the electrodes treated with different concentrations of NaBH4 . c) Peaks separation as a function of the scan rate for the treated electrodes. d) Roughness factor of the films at the different concentrations of chemical agent. e) SWV response of the films treated with 1 m NaBH4 for concentrations of MB in PBS ranging from 100 nM to 1 mM. f) Calibration curve fitted with a Hill equation (inset) for the films treated with the different concentrations of the sintering agent (n = 3). SWV parameters: 25 Hz frequency, 25 mV amplitude, potential range from 0.1 to −0.4 V. all the gold surface is active toward redox processes, and the click ceptable resolution and precision over Mitsubishi substrate and sintering at high concentrations enhances this aspect. Kapton (Figure S15, Supporting Information).[85] We verified the Finally, to approach the use of these electrodes as a substrate ink is stable inside and outside the printer up to one month, with for biosensing, methylene blue (MB) was also used on the dif- no sign of aggregation as shown from dynamic light scattering ferently treated gold surfaces (Figure 6d–f). MB was chosen as and UV–vis spectroscopy (the time was selected accounting our a model probe since it is widely used as mediator and redox re- average usage of ink for fabricating electrodes) (Figure S15, Sup- porter in biosensing devices.[56,84] There is a clear change in the porting Information). The simultaneous printing of AgNP inks performance of the film once the sintering agent threshold con- for the reference electrodes and the contacts allow to perform the centration is reached (Figure S11, Supporting Information). The complete NPs film deposition in a single print (lasting less than EC proprieties of the material change just slightly with a further 5 min). The sintering step is then performed in less than 10 min increase of the concentration of NaBH4 due to a more efficient de- per page with the possibility to use the same solution for more capping, rather than more extended nanostructural changes. The pages. The washing is performed first in deionized water and af- click-sintered electrodes allows to detect MB in solution in the terward in milliQ water for less than 2 min. The method as it is low μm range using SWV, without any additional post-treatment. can thus be easily scaled-up. A further improvement in this di- rection can be performed by adding the sintering agent as an ink in the inkjet printer and printing it on top of the gold electrodes 2.4. Scalability, Integration, and Comparison in a second passage. We have verified that the alignment grants a good coverage of the electrodes and the waste of reagent is drasti- The method described in this work is divided in three steps: 1) cally reduced even if more than one passage is needed for a com- ink loading; 2) film printing; 3) click sintering and washing. After plete sintering (as shown in Figure 2b). With this method, despite the first step, 6 mL of the AuNP ink can be printed for the fab- the obvious advantages in terms of volume of sintering agent and rication of up to 15–20 pages, each one containing around 130 precision of its deposition, the stability over time of the sintering electrochemical devices (Figure S14, Supporting Information). agent in the consumer printer may be affected by eventual cross The rheological parameters, have been selected and modified ac- contaminations onto the printhead by residues of the AuNP ink. cording to a previously published work using commercial silver For this reason, the complete integration for further scaling-up nanoparticles (AgNPs) ink, and proven to be printable with ac- of the method would be better achieved with ad-hoc designed Small 2023, 2306167 2306167 (9 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com printers, preventing the ink mixing in the printhead cleaning concentration including 1.6 mm PVP capping agent in a printing solution phase, or simply by using two printers, the second one being ex- composed of water (72%), ethylene glycol (25%), ethanol (2%), and iso- clusively dedicated to the sintering agent. In Table 1, we compare propyl alcohol (1%). Device Fabrication: The gold inkjet-printed lines were fabricated us- the results of our method with the main state-of-the-art proto- ing an Epson XP15000 consumer printer loaded with the 15% w/v AuNP cols for the formation of nanoporous gold, and how our method nanoink. Six milliliters of the ink were loaded into an empty cartridge (sup- drastically simplifies the process to obtain it in an additive, sus- plier) already installed on the printer printhead (cyan channel). After the tainable, easy, and rapid way at room temperature. initial printhead cleaning, the observers waited one day before perform- ing the nozzle check and starting the printing of the devices. Two different substrates were used for printing: transparent polyethylene terephthalate 3. Conclusion (PET) and Kapton. PET sheets in A4 dimension had a porous coating to allow the immediate drying of the AuNPs solvent and optimal adhesion We introduced a novel sintering method that we call click- (Mitsubishi Paper Mills NB-TP-3GU100). Both the printer and the sub- sintering, which enhances and tunes the nanostructuring of strate were selected in previous works.[85,88] Kapton sheets (Kapton KPC) AuNPs films at room temperature. This unique and innovative were purchased from DuPont. The substrate was cut into A5 sheets and strategy utilizes the internal energy produced during a catalytic treated with 4 m NaOH to increase the surface wettability. The printed patterns were designed in Autodesk AutoCAD 2020 and were composed exothermic reaction to induce the coalescence of NPs, resulting of two connection pads (3 mm x 3 mm) and 2 cm long 600 μm wide lines in a web-like nanostructured conductive gold surface. We pro- connecting them. pose a possible implementation of this technique integrating it Click Sintering of AuNPs Films–Immersion Method: Sodium borohy- into a consumer inkjet printer, providing an affordable strategy dride powder (purity 98% and MW of 37.83 g mol−1 ) and sodium for one-step printing of nanostructured conductive substrates, hydroxide pellet (purity>98%, 40.05 g mol−1 MW) were purchased without any post-treatment and opening the way to the use of from Merck, Sigma–Aldrich. Ultrapure MilliQ Millipore deionized water (18.2 MΩ cm−1 ) was used for the preparation of the solutions. Sintering sensitive and fragile substrates since no thermal treatments are solutions were prepared dissolving NaBH4 0.001, 0.01, 0.1, 0.5, and 1 m needed after the film deposition. The potential applications for in 0.5% sodium hydroxide to stabilize the sintering agent and slow down this technology are vast, including flexible electronics, energy, the decomposition reaction.[62,63] The films were transferred to glass Petri catalysis, and biosensing. We have thoroughly investigated the dishes immediately after preparation and immersed in the sintering solu- modulation mechanisms of the substrate’s electrochemical prop- tion for the selected time (10 min if not otherwise specified) at room tem- erties mediated by the sintering process. Our findings show perature under shaking. The films were then dipped twice in Milli-Q water that the modes of coalescence behind the net-like structure are to stop the reaction and remove potential side products, dried with a towel to remove the water excess and with nitrogen until completely dried. related to the de-capping mechanism of the polymer and the Inkjet Real-Time Click Sintering: Consumer printers had multi-ink rates of how this phenomenon happens is strictly correlated to (color) cartridges connected to a single printhead with separated nozzle the nanochemical environment in the interphases between the arrays. The loading of the AuNPs in a color channel (e.g., cyan) and of the particles. This study provides new insights about the complex sintering agent in a second one (e.g., yellow) allows for the implementa- sintering mechanism puzzle and opens alternative possibilities tion of sintering of the printed AuNPs by printing the desired designs in for the direct and straightforward fabrication and modulation mixed colors (e.g., green). The sintering ink was prepared by dissolving sodium borohydride in a solution containing 72% NaOH solution 0.5 m, of nanofunctional structures using an easy, cheap, and rapid 21% ethylene glycol, 2% Ethanol, and 1% isopropyl alcohol, to reach a fi- methodology. nal concentration of 0.5 m. The ink was loaded in the cartridge, which was inserted in the yellow channel housing, performing multiple prints on the gold films, drying after each printing, and performing the washing only af- ter the desired number of layers were deposited. This was done for two 4. Experimental Section reasons: first to increase the local concentration of the agent before, and AuNPs Ink Synthesis: Ultrapure, water-based AuNPs inks were syn- second to mimic the situation of real-time printing and sintering where the thesized via pulsed laser ablation in liquids by RHP-Technology with a sheets undergo multiple passes before being washed or used. The gold single-step method enabling the synthesis of size-controlled and highly- films, for sake of simplicity, were cut and attached to a paper sheet (Figure stable colloids at room temperature and atmospheric pressure.[86,87] The S3, Supporting Information) to be placed inside the printer. laser system included an InnoSLab, nanosecond pulsed laser (IS400, Ink Properties Characterization–Rheological Properties: The viscosity of Edgewave) delivering 7 ns full-half width maximum (FHWM) pulses at the ink was measured using an AMETEK Brookfield Viscometer for low 1064 nm, at a repetition rate of maximum 10 kHz and pulse energy of torque ranges (CZ-40 spindle), performing a full scan at different shear 40 mJ. The laser beam was focused by a high-speed Galvo scanner (in- speeds and fitting a linear curve extracting the viscosity from the slope telliSCAN III-10, Scanlab) through a telecentric 65 mm focal length F-𝜃 of the curve. The measurements of the contact angle and the surface ten- quartz lens (S4LFT4065/328, Sill Optics). Spherical-shaped AuNPs with sion were performed by means of a Drop Shape Analyzer DSA25S (KRÜSS a mean diameter of 15 ± 5 nm were generated from a 99.99% pure GmbH, Hamburg, Germany), using the sessile drop and pendant drop gold target (Agosi AG) in a closed flow chamber whereas the laser beam techniques, respectively. The drop shapes were analyzed in both cases with was coupled horizontally into the process chamber. As liquid media, 1 L the KRÜSS ADVANCE software. For the analysis of the contact angles, Type I. water (>18 MΩ cm, Milli-Q, Merck-Millipore) containing 0.01 mm drops of 2 μL were cast on the surfaces with an automatically software- polyvinylpyrrolidone (PVP10, Sigma Aldrich) was used, while a controlled controlled syringe and 0.5 mm diameter steel needles. In the case of liquid flow of 500 mL min−1 was generated by a non-contact tube pump the surface tension, the pendant drops were generated by an automatic (Heidolph, Hei-FLOW Precision 06) circulating the liquid between a liquid software-controlled dosser in the tip of a 1.8 mm diameter steel needle. reservoir and the ablation chamber. Up-concentration of the gold colloid Dynamic Light Scattering (DLS): DLS and zeta potential measure- into a highly-stable nanoink was achieved by rotary evaporation (Hei-VAP ments were performed with Zeta Sizer Nano ZS (Malvern Instruments), in Precision, Heidolph) (35 °C, 35 mBar, 140 rpm) to remove the excess water disposable Zeta Sizer cuvettes using the automatic focus calibration and and to adjust the NP concentration. A water-based gold-ink with 26.5 wt.% analysis. The measurements were performed every week for 1 month dilut- solid particle content was thus obtained, that showed excellent long-term ing 1 μL of the prepared ink into 1000 μL of water or water/ethylene glycol stability. The nanoink formulation was then adjusted to 15 wt.% AuNP dispersion. Small 2023, 2306167 2306167 (10 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com UV–Vis: Absorbance spectra were acquired using a Plate Reader Spec- Electrochemical Characterization: For all the electrochemical measure- tramax I3, scanning from 400–800 nm with a 5 nm step, using an ELISA ments, one connection pad of the described layout (see Device fabrica- plate filled with 100 μL of different concentrations of gold ink diluted in tion) had served as contact and the other as a working electrode. A repro- MillQ water. A blank spectra of the MilliQ water was acquired and used as ducible working electrode area was obtained insulating the AuNPs connec- a baseline. tion line (between the two pads) with screen printing insulating paste. The Surface Characterizations: Pictures of the printed lines were acquired three-electrode electrochemical cell was completed with an external stan- by an Andostar ADSM302 optical microscope and a Nikon microscope dard reference (Ag/AgCl 3 m KCl) and counter (platinum wire) electrodes Eclipse using the 5x lenses, keeping the same focus and brightness dur- (CH Instruments). All the reagents for the redox probe solutions were pur- ing the whole study. The line width was calculated using ImageJ software, chased from Merck (Sigma–Aldrich) and prepared as following: equimo- calibrating the scale using a SEM image of a TEM grid. lar quantities (5 mm) of potassium ferrocyanide (K4 [Fe(CN)6 ]·3H2 O) and The SEM analysis was performed by an XHRSEM Magellan 400L potassium ferricyanide (K3 [Fe(CN)6 ]) were dissolved in an appropriate vol- Thermo Fisher Scientific microscope. The images were collected operating ume of PBS, pH 7.4. Methylene blue solution was prepared dissolving the in a high vacuum with an electron beam energy of 2 kV and secondary elec- MB into PBS, obtaining the mother solution with a formal concentration trons in-lens detector. The EDX mapping was performed right after taking of 5 mm. The lower concentrations were obtained by serial dilution of the the pictures increasing the energy to 6 kV and looking for gold, carbon, mother solution. oxygen, nitrogen, and sodium elements. The electrochemical characterizations were performed using a Palm- Transmission electron microscopy (TEM) images and energy disper- Sens4 potentiostat and Ivium Octostat. CVs were performed in 5 mm sive X-ray (EDX) spectra were acquired on an FEI Tecnai G2 F20 HRTEM ferro/ferricyanide in PBS, scanning the potential from −0.35 to 0.65 V with coupled to an EDAX super ultra-thin window (SUTW) X-ray detector. scan rates of 25, 49, 64, 81, 100, 144, 196, 289, 400, and 484 mV s −1 . The The XPS analysis was performed at room temperature by a SPECS CVs in 0.1 M H2 SO4 were performed using a scan rate of 100 mV s −1 PHOIBOS 150 hemispherical analyzer (SPECS GmbH, Berlin, Germany) using a scanning potential from 0 to 1.55 V. with a base pressure of 5 10−10 mbar using monochromatic A1 K alpha Square wave voltammetry (SWV) was performed to obtain the methy- radiation (1486.74 eV) as excitation source, with a power of 300 W. The en- lene blue (MB) curve for the treated devices. SWV parameters used: equi- ergy resolution was 0.62 eV, as measured by the full width at half maximum libration time 10 s at 0.1 V, frequency 25 Hz, amplitude 25 mV, potential (FWHM) of the Ag 3d5/2 peak for sputtered silver foil. The spectra were ranging from 0.1 to −0.5 V (vs Ag/AgCl), E step of 0.003 mV. The concen- calibrated using C 1s at 284.8 eV as reference. The data analysis and fit- tration of MB ranged from 100 nm to 1 mm. ting were performed using CasaXPS software. The constructed peak model was defined by using a Voigt function, with a Shirley background for the Au 4f. The peak model for the carbon peak was defined using a Gaussian- Lorentzian function, with no constraints on the peak position and keeping Supporting Information constant the ratio between the peak areas of the components. Supporting Information is available from the Wiley Online Library or from The SERS measurements were performed by using a Renishow inVia the author. confocal micro-Raman system, coupled with an optical Leica DLML mi- croscope equipped with a NPLAN objective 20x with a numerical aperture of 0.75. The laser source used was a He-Ne laser at 633 nm. The spec- trometer consists of a single grating monochromator (1800 lines mm−1 ), Acknowledgements coupled with a CCD detector, a RenCam 578×400 pixels (22 μm x 22 μm) cooled by a Peltier element. Spectral resolution of the spectrometer was The ICN2 is funded by the CERCA programme/Generalitat de 2.0 cm−1 . On the electrode surface was drop casted a Rhodamine B (pur- Catalunya. The ICN2 is supported by the Severo Ochoa Centres chased by Sigma Aldrich/Merck) solution 1*10−6 M in MilliQ water. of Excellence programme, Grant CEX2021-001214-S, funded by Electrical Characterizations–2-Point Probe Setup: The electrical resis- MCIN/AEI/10.13039.501100011033. This project has received fund- tance of the AuNPs printed lines was measured by a Keithley DMM6500 ing from the European Union’s Horizon 2020 research and innovation multimeter with standard probes, simply touching the pads at the two ex- programme under grant agreement No 101008701. Views and opinions tremes of each line. expressed were however those of the author(s) only and did not necessar- 4-Point Probe Setup: In this work, pogo pin-type equidistant probes ily reflect those of the European Union. The European Union could not be were used with a pressure of 120 g and a separation between them of held responsible for them. PID2021-124795NB-I00 funded by MCIN/AEI/ 2.54 mm. A requirement for this technique was that the thickness of the 10.13039/501100011033 and by “ERDF A way of making Europe”. This film was less than 0.4 times the separation of the probes, which corre- work has been carried out within the framework of the doctoral pro- sponds in this case to less than 1.016 mm. The probes were centered in gram ‘Doctorado en Biotecnologia’ from the Universitat Autonoma de the area to be measured (Figure S4, Supporting Information). Since the ra- Barcelona. G.M. acknowledges the Fundación Carolina for the Doctoral tio between the side of the square (W) and the separation between probes Scholarship granted under the program “Doctorado 2020”. F.D.P. ac- (S) was less than 40, W/S = 20/2.54 = 7.87, it was necessary to apply a knowledge the Ministry of Education, University and Research (MIUR) geometric correction reported in the literature (Haldor Topsøe, in Geomet- and European Social Fund (ESF) for the PON R&I 2014–2020 program, ric Factors in Four Point Resistivity Measurement 1966). In this work, the action 1.2 “AIM: Attraction and International Mobility” (AIM1894039-3). correction factor used (C) for a W/S = 7.5 was C = 0.8846. Under these A.B. is supported by the ERASMUS+ mobility program during the doctoral conditions, the sheet resistivity Rsq was given by Equation 1: school in Chemistry and Material Science (DSCM) at the Department of Chemistry and Industrial Chemistry, University of Pisa (UNIPI), Italy. ( ) ( ) 𝜋 ΔV ΔV Rsq = ⋅C⋅ =4 (1) ln (2) I I Conflict of Interest where (ΔV/I) is the value reported by the multimeter. The authors declare no conflict of interest. AFM of the Thickness: AFM images were acquired using a PicoSPM II atomic force microscope in tapping mode. Images were processed using Gwyddion software for obtaining the profiles, exported, plotted, and pro- Data Availaility Statement cess on Origin. Values obtained for the thickness were obtained by sub- tracting the average of the base points from the average of the top points, The data that support the findings of this study are available from the cor- for four total profiles. responding author upon reasonable request. Small 2023, 2306167 2306167 (11 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Keywords [30] G. Hu, T. Albrow-Owen, X. Jin, A. Ali, Y. Hu, R. C. T. Howe, K. Shehzad, Z. Yang, X. Zhu, R. I. Woodward, T.-C. Wu, H. Jussila, J.-B. Wu, P. Peng, flexible substrate, gold nanoparticles, inkjet printing, nanoporous gold, P.-H. Tan, Z. Sun, E. J. R. Kelleher, M. Zhang, Y. Xu, T. Hasan, Nat. nanostructures, sintering Commun. 2017, 8, 278. [31] C. J. Zhang, L. McKeon, M. P. Kremer, S. H. Park, O. Ronan, A. Seral- Received: July 21, 2023 Ascaso, S. Barwich, C. Coileáin, N. McEvoy, H. C. Nerl, B. Anasori, Revised: September 26, 2023 J. N. Coleman, Y. Gogotsi, V. Nicolosi, Nat. Commun. 2019, 10, Published online: 1795. [32] G. Rosati, M. Ravarotto, M. Scaramuzza, A. De Toni, A. Paccagnella, Sensors Actuators, B Chem. 2019, 280, 280. [33] J. Frenkel, J. Phys. 1945, 9, 385. [1] M. Faraday, Philos. Trans. R. Soc. 1857, 147, 145. [34] Y. Ge, X. Duan, M. Zhang, L. Mei, J. Hu, W. Hu, X. Duan, J. Am. Chem. [2] A. Corma, H. Garcia, Chem. Soc. Rev. 2008, 37, 2096. Soc. 2018, 140, 193. [3] , Chem. Rev. 2012, 112, 2739. [35] J. Liu, Y. Ge, D. Zhang, M. Han, M. Li, M. Zhang, X. Duan, Z. Yang, J. [4] E. C. Dreaden, A. M. Alkilany, X. Huang, C. J. Murphy, M. A. El-Sayed, Hu, Nano Mater 2021, 4, 1664. Chem. Soc. Rev. 2012, 41, 2740. [36] G. Maroli, S. Boyeras, H. Giannetta, S. Pazos, J. Gak, A. R. Oliva, M. [5] H. Chen, L. Shao, Q. Li, J. Wang, Chem. Soc. Rev. 2013, 42, 2679. A. Volpe, P. M. Julian, F. Palumbo, Front. Electron. 2023, 3, 1060197. [6] Y.-C. Yeh, B. Creran, V. M. Rotello, Nanoscale 2012, 4, 1871. [37] J. R. Greer, R. A. Street, Acta Mater. 2007, 55, 6345. [7] C. Parolo, A. Sena-Torralba, J. F. Bergua, E. Calucho, C. Fuentes-Chust, [38] S. Wünscher, R. Abbel, J. Perelaer, U. S. Schubert, J. Mater. Chem. C L. Hu, L. Rivas, R. Álvarez-Diduk, E. P. Nguyen, S. Cinti, D. Quesada- 2014, 2, 10232. González, A. Merkoçi, Nat. Protoc. 2020, 15, 3788. [39] H. W. Tan, J. An, C. K. Chua, T. Tran, Adv. Electron. Mater. 2019, 5, [8] C. Fuentes-Chust, C. Parolo, G. Rosati, L. Rivas, K. Perez-Toralla, S. 1800831. Simon, I. de Lecuona, C. Junot, J. Trebicka, A. Merkoçi, Adv. Mater. [40] S.-H. Park, H.-S. Kim, Thin Solid Films 2014, 550, 575. 2021, 33, 2006104. [41] J. Perelaer, R. Abbel, S. Wünscher, R. Jani, T. Van Lammeren, U. S. [9] G. Rosati, A. Idili, C. Parolo, C. Fuentes-Chust, E. Calucho, L. Hu, C. Schubert, Adv. Mater. 2012, 24, 2620. D. C. Castro E Silva, L. Rivas, E. P. Nguyen, J. F. Bergua, R. Alvárez- [42] W. Gu, W. Yuan, T. Zhong, X. Wu, C. Zhou, J. Lin, Z. Cui, RSC Adv. Diduk, J. Muñoz, C. Junot, O. Penon, D. Monferrer, E. Delamarche, 2018, 8, 30215. A. Merkoçi, ACS Nano 2021, 15, 17137. [43] Y. Sui, Y. Dai, C. C. Liu, R. M. Sankaran, C. A. Zorman, Adv. Mater. [10] J. B. Chen, H. Yousefi, C. R. Nemr, S. Gomis, R. Atwal, M. Labib, E. Technol. 2019, 4, 1900119. Sargent, S. O. Kelley, Adv. Funct. Mater. 2020, 30, 1907701. [44] S. Jang, H. Cho, S. Kang, S. Oh, D. Kim, Appl. Phys. A Mater. Sci. [11] A. N. Koya, X. Zhu, N. Ohannesian, A. A. Yanik, A. Alabastri, R. Proietti Process. 2011, 105, 685. Zaccaria, R. Krahne, W.-C. Shih, D. Garoli, ACS Nano 2021, 15, 6038. [45] D. D. Liana, B. Raguse, L. Wieczorek, G. R. Baxter, K. Chuah, J. J. [12] X. Lang, A. Hirata, T. Fujita, M. Chen, Nat. Nanotechnol. 2011, 6, 232. Gooding, E. Chow, RSC Adv. 2013, 3, 8683. [13] J. Wang, Microchim. Acta 2012, 177, 245. [46] T. S.-W. Leung, E. Ramon, C. Martínez-Domingo, Adv. Eng. Mater. [14] C. Hu, X. Bai, Y. Wang, W. Jin, X. Zhang, S. Hu, Anal. Chem. 2012, 84, 2023, 25, 2200834. 3745. [47] B. T. Anto, S. Sivaramakrishnan, L.-L. Chua, P. K. H. Ho, Adv. Funct. [15] A. Määttänen, P. Ihalainen, P. Pulkkinen, S. Wang, H. Tenhu, J. Mater. 2010, 20, 296. Peltonen, ACS Appl. Mater. Interfaces 2012, 4, 955. [48] A. Gupta, S. Mandal, M. Katiyar, Y. N. Mohapatra, Thin Solid Films [16] F. Jia, C. Yu, Z. Ai, L. Zhang, Chem. Mater. 2007, 19, 3648. 2012, 520, 5664. [17] Y. Deng, W. Huang, X. Chen, Z. Li, Electrochem. Commun. 2008, 10, [49] L.-K. Lin, J.-T. Tsai, S. Díaz-Amaya, M. R. Oduncu, Y. Zhang, P.-Y. 810. Huang, C. Ostos, J. P. Schmelzel, R. Mohammadrahimi, P. Xu, A. M. [18] K. Murata, K. Kajiya, M. Nukaga, Y. Suga, T. Watanabe, N. Nakamura, Ulloa Gomez, S. N. Shuvo, N. Raghunathan, X. Zhang, A. Wei, D. H. Ohno, Electroanalysis 2010, 22, 185. Bahr, D. Peroulis, L. A. Stanciu, ACS Appl. Mater. Interfaces 2021, 13, [19] M. Saito, A. Kitamura, M. Murahashi, K. Yamanaka, L. Q. Hoa, Y. 11369. Yamaguchi, E. Tamiya, Anal. Chem. 2012, 84, 5494. [50] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, ACS Nano [20] D.-P. Song, A. Naik, S. Li, A. Ribbe, J. J. Watkins, J. Am. Chem. Soc. 2010, 4, 1943. 2016, 138, 13473. [51] Z. Li, K. S. Suslick, Angew. Chemie. Int. Ed. 2019, 58, 14193. [21] L. P. Hernández-Saravia, A. Sukeri, M. Bertotti, Int. J. Hydrogen Energy [52] M. J. Coutts, M. B. Cortie, M. J. Ford, A. M. Mcdonagh, J. Phys. Chem. 2019, 44, 15001. C 2009, 113, 1325. [22] N. Xing, L. Lian, Y. Liu, Y. Shi, Mater. Lett. 2019, 254, 125. [53] D. Tobjörk, H. Aarnio, P. Pulkkinen, R. Bollström, A. Määttänen, [23] P. Varasteanu, C. Romanitan, A. Bujor, O. Tutunaru, G. Craciun, I. P. Ihalainen, T. Mäkelä, J. Peltonen, M. Toivakka, H. Tenhu, R. Mihalache, A. Radoi, M. Kusko, Nanomaterials 2020, 10, 2321. Österbacka, Thin Solid Films 2012, 520, 2949. [24] S. Nasrollahi, S. M. Ghoreishi, A. Khoobi, J. Electroanal. Chem. 2020, [54] S. H. Ko, H. Pan, C. P. Grigoropoulos, C. K. Luscombe, J. M. J. Fréchet, 864, 114097. D. Poulikakos, Appl. Phys. Lett. 2007, 90, 141103. [25] J. A. Hondred, Z. T. Johnson, J. C. Claussen, J. Mater. Chem. C 2020, [55] H. Yamauchi, T. Dobashi, S. Sato, Chem. Lett. 2012, 41, 1154. 8, 11376. [56] A. M. Downs, J. Gerson, M. N. Hossain, K. Ploense, M. Pham, H.-B. [26] Y. Xu, X. Wang, H. Chen, L. Chen, W. Chen, X. Yin, A. Liu, X. Lin, S. Kraatz, T. Kippin, K. W. Plaxco, ACS Sens. 2021, 6, 2299. Weng, Y. Zheng, J. Electroanal. Chem. 2021, 886, 115109. [57] S. M. Ansar, F. S. Ameer, W. Hu, S. Zou, C. U. Pittman, D. Zhang, [27] P. Sondhi, D. Neupane, J. K. Bhattarai, A. V. Demchenko, K. J. Stine, Nano Lett. 2013, 13, 1226. J. Electroanal. Chem. 2022, 924, 116865. [58] G. S. Perera, S. M. Ansar, S. Hu, M. Chen, S. Zou, C. U. Pittman, D. [28] Q. Wan, J. Li, Z. Liu, L. Han, S. Huang, Z. Wang, Metals 2023, 13, Zhang, J. Phys. Chem. C 2014, 118, 10509. 1255. [59] H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, H. R. [29] D. Mcmanus, S. Vranic, F. Withers, V. Sanchez-Romaguera, M. Hoekstra, E. K. Hyde, J. Am. Chem. Soc. 1953, 75, 215. Macucci, H. Yang, R. Sorrentino, K. Parvez, S.-K. Son, G. Iannaccone, [60] R. Peña-Alonso, A. Sicurelli, E. Callone, G. Carturan, R. Raj, J. Power K. Kostarelos, G. Fiori, C. Casiraghi, Nat. Nanotechnol. 2017, 12, 343. Sources 2007, 165, 315. Small 2023, 2306167 2306167 (12 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306167 by Readcube (Labtiva Inc.), Wiley Online Library on [16/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com [61] H. N. Abdelhamid, Int. J. Hydrogen Energy 2021, 46, 726. [77] J. Langer, D. Jimenez De Aberasturi, J. Aizpurua, R. A. Alvarez-Puebla, [62] Y. Shang, R. Chen, Energy Fuels 2006, 20, 2142. B. Auguié, J. J. Baumberg, G. C. Bazan, S. E. J. Bell, A. Boisen, A. G. [63] V. G. Minkina, S. I. Shabunya, V. I. Kalinin, V. V. Martynenko, A. L. Brolo, J. Choo, D. Cialla-May, V. Deckert, L. Fabris, K. Faulds, F. J. Smirnova, Int. J. Hydrogen Energy 2012, 37, 3313. García De Abajo, R. Goodacre, D. Graham, A. J. Haes, C. L. Haynes, [64] D. Bhattacharjee, S. Dasgupta, J. Mater. Chem. A 2015, 3, 24371. C. Huck, T. Itoh, M. Käll, J. Kneipp, N. A. Kotov, H. Kuang, E. C. Le [65] S. Kumar, A. Jain, H. Miyaoka, T. Ichikawa, Y. Kojima, Int. J. Hydrogen Ru, H. K. Lee, J.-F. Li, X. Y. Ling, et al., ACS Nano 2020, 14, 28. Energy 2017, 42, 22432. [78] P. Hildebrandt, M. Stockburger, J. Phys. Chem. 1984, 88, 5935. [66] X. Zhou, W. Xu, G. Liu, D. Panda, P. Chen, J. Am. Chem. Soc. 2010, [79] F. Tian, F. Bonnier, A. Casey, A. E. Shanahan, H. J. Byrne, Anal. Meth- 132, 138. ods 2014, 6, 9116. [67] J. C. Hoogvliet, M. Dijksma, B. Kamp, W. P. Van Bennekom, Anal. [80] M. Suzuki, Y. Niidome, Y. Kuwahara, N. Terasaki, K. Inoue, S. Yamada, Chem. 2000, 72, 2016. J. Phys. Chem. B 2004, 108, 11660. [68] S. P. Chenakin, N. Kruse, Phys. Chem. Chem. Phys. 2016, 18, 22778. [81] S.-Y. Ding, J. Yi, J.-F. Li, B. Ren, D.-Y. Wu, R. Panneerselvam, Z.-Q. [69] Y. Borodko, S. E. Habas, M. Koebel, P. Yang, H. Frei, G. A. Somorjai, Tian, Nat. Rev. Mater. 2016, 1, 16021. J. Phys. Chem. B 2006, 110, 23052. [82] R. L. Bard, L. R. Faulkner, H. S. White, Electrochemical Methods: Fun- [70] Y. Borodko, S. M. Humphrey, T. D. Tilley, H. Frei, G. A. Somorjai, J. damental and Applications, John Wiley & Sons, Hoboken, New Jersey Phys. Chem. C 2007, 111, 6288. 2001. [71] I. Miguel-García, Á. Berenguer-Murcia, T. García, D. Cazorla-Amorós, [83] M. M. Collinson, Hindawi 2013, https://doi.org/10.1155/2013/ Catal. Today 2012, 187, 2. 692484. [72] J.-Y. Ye, G. A. Attard, A. Brew, Z.-Y. Zhou, S.-G. Sun, D. J. Morgan, D. [84] J. Liu, S. Wagan, M. Dávila Morris, J. Taylor, R. J. White, Anal. Chem. J. Willock, J. Phys. Chem. C 2016, 120, 7532. 2014, 86, 11417. [73] J. García-Aguilar, M. Navlani-García, Á. Berenguer-Murcia, K. Mori, [85] G. Rosati, M. Urban, L. Zhao, Q. Yang, C. De Carvalho Castro E Silva, Y. Kuwahara, H. Yamashita, D. Cazorla-Amorós, Langmuir 2016, 32, S. Bonaldo, C. Parolo, E. P. Nguyen, G. Ortega, P. Fornasiero, A. 12110. Paccagnella, A. Merkoçi, Biosens. Bioelectron. 2022, 196, 113737. [74] J. Xian, Q. Hua, Z. Jiang, Y. Ma, W. Huang, Langmuir 2012, 28, 6736. [86] C. L. Sajti, R. Sattari, B. N. Chichkov, S. Barcikowski, J. Phys. Chem. C [75] G. Ruffato, F. Romanato, D. Garoli, S. Cattarin, Opt. Express 2011, 19, 2010, 114, 2421. 13164. [87] C. L. Sajti, A. Barchanski, P. Wagener, S. Klein, S. Barcikowski, J. Phys. [76] D. M. O. of N.-B. S. S. through L.-S. R. S. Solís, J. M. Taboada, F. Chem. C 2011, 115, 5094. Obelleiro, L. M. Liz-Marzán, F. J. García De Abajo, ACS Photonics [88] G. Rosati, M. Ravarotto, M. Sanavia, M. Scaramuzza, A. De Toni, A. 2017, 4, 329. Paccagnella, Sens. Bio-Sensing Res. 2019, 26, 100308. Small 2023, 2306167 2306167 (13 of 13) © 2023 The Authors. Small published by Wiley-VCH GmbH