pubs.acs.org/acssensors Article One-Step Laser Nanostructuration of Reduced Graphene Oxide Films Embedding Metal Nanoparticles for Sensing Applications Annalisa Scroccarello,# Ruslan Á lvarez-Diduk,*,# Flavio Della Pelle, Cecilia de Carvalho Castro e Silva, Andrea Idili, Claudio Parolo, Dario Compagnone, and Arben Merkoçi* Cite This: ACS Sens. 2023, 8, 598−609 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The combination of two-dimensional materials and metal nano- particles (MNPs) allows the fabrication of novel nanocomposites with unique physical/chemical properties exploitable in high-performance smart devices and biosensing strategies. Current methods to obtain graphene-based films decorated with noble MNPs are cumbersome, poorly reproducible, and difficult to scale up. Herein, we propose a straightforward, versatile, surfactant-free, and single-step technique to produce reduced graphene oxide (rGO) conductive films integrating “naked” noble MNPs. This method relies on the instantaneous laser-induced co- reduction of graphene oxide and metal cations, resulting in highly exfoliated rGO nanosheets embedding gold, silver, and platinum NPs. The production procedure has been optimized, and the obtained nanomaterials are fully characterized; the hybrid nanosheets have been easily transferred onto lab-made screen-printed electrodes preserving their nanoarchitecture. The Au@rGO-, Ag@rGO-, and Pt@ rGO-based electrodes have been challenged to detect caffeic acid, nitrite, and hydrogen peroxide in model solutions and real samples. The sensors yielded quantitative responses (R2 ≥ 0.997) with sub-micromolar limits of detections (LODs ≤ 0.6 μM) for all the analytes, allowing accurate quantification in samples (recoveries ≥ 90%; RSD ≤ 14.8%, n = 3). This single-step protocol which requires low cost and minimal equipment will allow the fabrication of free-standing, MNP-embedded rGO films integrable into a variety of scalable smart devices and biosensors. KEYWORDS: laser reduced graphene oxide, IR-laser, nanodecoration, micropatterning, hybrid nanomaterials, surfactant-free, electrochemical sensor ■ INTRODUCTION Nanomaterials (NMs) have recently become major components the implementation of these hybrid NMs outside the research laboratories is still very limited. Overall, the methods to synthesize NMs can be categorized of new electronic/optical smart devices and (bio)sensors into bottom-up and top-down approaches. The former consists because of the unique physical/chemical features that matter of assembling atoms into ordered nanostructures requiring exhibits at the nanometer scale.1,2 During the last two decades, accurate control of all reaction conditions (e.g., substrate metal nanoparticles (MNPs), thanks to their optical, mechan- concentration, pH, and temperature), while the latter consists of ical, thermal, electrical, and physical properties, have been breaking down bulk materials into nanosized structures. commonly used in several rapid diagnostic tests (e.g., lateral flow Bottom-up approaches are generally used for the synthesis of assays and fluorescent assays) and electronic appliances (e.g., MNPs; on the other hand, top-down approaches are generally smart-TVs).1,3 Similarly, two-dimensional (2D) materials, in more convenient for the fabrication of 2D materials (e.g., particular, graphene, having a unique surface-to-volume ratio graphene) since they provide higher yield and an easier and conductive properties, are also used in several (bio)sensing procedure.13−16 Studies aiming at fabricating NCs made of 2D and smart devices. More recently, the research community has materials and MNPs rely on the separate synthesis of NMs and been focused on studying the properties of multicomponent and their combination via physical adsorption or chemical nanostructured materials with at least one dimension in the nanometer scale. In this respect, nanocomposites (NCs) originating from 2D materials, in particular, graphene/graphene Received: August 17, 2022 derivatives (i.e., graphene oxide) and MNPs, proved to possess Accepted: November 8, 2022 useful features; these have been exploited to develop sensing Published: February 3, 2023 substrates for optical,4,5 electrochemical,6−9 and gas10−12 sensors that have been successfully used for different applications. Despite their peculiar and promising properties, © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acssensors.2c01782 598 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article interactions.17−19 It is also possible to perform the in situ synthesis of MNPs onto the hosting NM (e.g., reduction of ■ MATERIALS AND METHODS Reagents and Stock Solution. A detailed description of all the metal ions mediated by an external reducing agent or by the reagents and stock solutions used is reported in the Supporting substrate itself, galvanic displacement, etc.).19−21 Although Information (Section S1.1). these two strategies generally guarantee effective nanodecora- Apparatus. The laser plotter employed was a Rayjet50 laser tions, they also imply three main limitations: (i) the use of machine from Trotec (Wels, Austria) equipped with a CO2 laser (10.6 μm, 30 W, laser spot of 0.04 mm). The nanodecorated film patterns external reducing compounds and capping agents that interact were designed with Corel Draw software. Vacuum filtration was intimately with the MNP surface to stabilize them, often performed through a 1 L vacuum-filtering flask, equipped with a 300 mL masking their properties as their electrical/electrochemical glass filter holder and 47 mm SS screen (1/Pk) from Millipore and a properties; (ii) limited control over the decoration pattern and vacuum pump from KNF LAB LABOPORT. Press transfer and MNP distribution; and (iii) needing cumbersome and multistep electrode screen printing were carried out using a laboratory hydraulic procedures, making the whole process costly and time- press and a screen printer machine, respectively. Laser-Assisted MNPs@rGO Film Formation. Working solutions consuming and, therefore, not suitable for the majority of of 30 mM gold/Au(III), 15 mM silver/Ag(I), and 30 mM platinum/ commercial applications. Pt(II) salt were prepared in Milli-Q water and promptly used. To In this framework, in the last decade, laser-based processing produce the Mn+@GO film, 0.4 mL of a 10 mg mL−1 GO stock solution technologies demonstrated tremendous potential for the was diluted with 5 mL of the respective metal salt solution; the patterning, structuring, and property tuning of NMs (e.g., dispersion was then stirred for 5 min and filtered onto a polyvinylidene doping, reduction, etc.).22−29 In this context, different laser fluoride (PVDF) membrane (0.1 μm pore size and 47 mm diameter). sources have been employed, among which the IR-laser is the The filtered material film was let to dry at room temperature for 20 min. After drying, the film was aligned according to the patterned design and most appealing due to the emitted energy, the low cost, and ease treated with the CO2 laser under a 7.4 cm focusing lens, using the of management. These aspects have bumped highly automated engraving mode (laser power 2.10 W and speed 1.50 m s−1). A pattern IR-laser plotters into the market. of 17 key-lock-like working electrodes (WE, Ø = 3 mm), obtained using The IR-laser has been strongly employed to process carbon- Corel Draw software, was used for “laser scribing”. A GO film formed based nanostructures; IR-laser-based technologies can produce with the same procedure without the metal source was used as the graphene by triggering photothermal events to carbon- control; the latter was produced by filtering 0.4 mL of 10 mg mL−1 GO stock solution diluted in 5 mL of Milli-Q water. containing substrates (i.e., hydrocarbon cyclic skeleton or Sensor Fabrication. The sensors were assembled to form complete aromatic heterocycles), inducing graphitization processes that three-electrode electrochemical cells with the laser-treated films used as consist of weak bond breaking and induction of sp2 nano- working electrodes (WE). structured domains.24,26−28 Although several studies report the The film (with the key-lock pattern) was cut in small rectangles use of the IR-laser to synthesize graphene or tune its properties, through the CO2 laser using the engraving mode (power 7.60 W and very few report the use of the IR-laser to simultaneously produce speed 0.75 m s−1). A polyethylene terephthalate (PET) sheet was used as the sensor base, and the PET was cleaned with a 95% ethanolic and nanodecorate the “graphene-based/like” structure with solution before use. The electrical contacts and the reference electrode MNPs.23,30 were screen-printed onto the PET sheet using conductive silver ink, Recently, Ye et al.30 carried out the first laser-induced whereas the counter electrode was made from carbon sensor paste. The graphene production and simultaneous decoration with metal WE pattern was aligned above the electrode surface according to the oxides, using a polyimide film embedded with metal complexes WE position. The nanostructured film was transferred onto the PET as precursors. You et al.23 fabricated interdigitated array substrate by applying a pressure of 3.0 ± 0.1 t for 3 min using a hydraulic electrodes exploiting the laser scribing patterning and ability press. Finally, the PVDF filter was removed, and the contacts were insulated using a dielectric insulating paint. The sensor component size to trigger graphitization from the polyimide film and to is reported in Figure S9. simultaneously reduce noble MNPs. In both cases, the laser Morphological and Chemical Characterization. For the treatment and MNP decoration occur in situ and are localized to morphological and elemental characterization of the generated the substrate’s shallow. Then, there is a clear need to investigate nanostructured films, a Magellan 400L XHR SEM (FEI, Hillsboro, new methods to obtain, in a single step, a graphene OR) instrument and a Quanta 650 FEG ESEM (FEI, Hillsboro, OR) nanostructure decorated with NMs easily transferable to any instrument were used for scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis. The MNP size substrate, allowing us to develop different types of (opto)- distribution (calculated on an average number of 200 MNPs) was electronic and electrochemical devices, also on a large scale. evaluated with “ImageJ, v.1.49.p”. X-ray photoelectron spectroscopy In this paper, we propose a strategy to produce, in a single (XPS) measurements were performed using a SPECS PHOIBOS 150 step, a freestanding film of reduced graphene oxide (rGO) hemispherical analyzer (SPECS GmbH, Berlin, Germany), with a base decorated with three different metal nanoparticles (AuNPs, pressure of 5 × 10−10 mbar, using a monochromatic Al K-alpha AgNPs, and PtNPs). The IR-laser was used to simultaneously radiation (1486.74 eV) as the excitation source. The XPS spectral deconvolution and the identification of the peaks were run�following and accurately produce a highly exfoliated rGO substrate and in the technique described by Sen et al.31�through the Gaussian− situ decorate it with naked MNPs. After presenting the careful Lorentzian fitting method, after smoothing and subtraction of the optimization of the method and the full characterization of the Shirley-shaped background. A confocal Raman spectrometer alpha300r nanocomposites, we also demonstrate how they can be (WITec, Ulm, German) equipped with a 488 nm laser, using a 1.5 mW successfully integrated into flexible substrates to fabricate laser power, a grating 600 g/nm, and objective 50×, was employed, functional electrodes. Finally, the resulting electrochemical using an exposure time of 10 s and three accumulations for each spectrum. sensors were used to detect caffeic acid, nitrite, and hydrogen Electrochemical Measurements. Electrochemical experiments peroxide in samples. Thanks to its low cost and ease of use, we were carried out using an AutoLab PGStat302 equipped with NOVA believe that our method can represent the solution to produce 2.1.3 (Metrohm AutoLab) software. The chemistry of the laser-formed high amounts of high-quality rGO films decorated with MNPs. nanostructured films was assessed by CV in 0.5 M H2SO4 for Au@rGO 599 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article and Pt@rGO, while in 0.1 M KCl for the Ag@rGO; in all the cases, a scan rate of 100 mV s−1 was adopted. The electrochemical characterization of the nanostructured sensors was carried out using cyclic voltammetry and electrochemical impedance spectroscopy using Fe(CN)62−/3− and Ru(NH3)63+/2+ at 3 mM in KCl 0.1 M as redox probes. EIS was conducted at open circuit potential, with a ±10 mV amplitude sinusoidal wave, scanning the 10−2 to 105 Hz frequency range. All the explorative CVs for both redox probes and analytes (caffeic acid, nitrite, and H2O2) were performed at 25 mV s−1. Differential pulse voltammetry (DPV) in 0.1 M phosphate buffer with 0.1 M KCl (PB) was employed to quantify caffeic acid (pH 7.0) and nitrite (pH 4.0), using a pulse width of 50 ms, a modulation amplitude of 50 mV, with a resulting scan rate of 25 mV s−1. Chronoamperometry was employed to quantify H2O2 in PB (pH 7.0), the measurement Figure 1. Sketch of the MNPs@rGO film formation and sensor encompassed an electrode preconditioning at +0.8 V (60 s) followed by fabrication. (A) GO mixing with metal precursor (Mn+); (B) Mn+@GO amperometry at 0.0 V, and the signal was recorded at 60 s. All the film formed after vacuum co-filtration of Mn+ and GO; (C) laser- electrochemical measurements were performed versus pseudo-Ag/ mediated reduction of the Mn+@GO film according to the working AgCl reference electrode in triplicate, placing a 100 μL drop of the electrode design; (D) formed MNPs@rGO-based working electrode solution to analyze on the top of the integrated electrochemical cell. laser cutting; (F) working electrode alignment and transfer onto the MNPs@rGO-Based Sensor for Caffeic Acid, NO2−, and H2O2 PET screen-printed electrode; (G) working electrode remaining’s Evaluation. Quantification of caffeic acid (CA) in a coffee sample and remotion; and (H) complete MNPs@rGO sensor. of nitrite in fishing lake water (Lago Paradiso, Mosciano Sant’Angelo, Teramo, Italy) was accomplished by DPV (same parameters as the Electrochemical Measurements section) using the Au@rGO- and Ag@ Figure 2A shows the SEM image of the rGO film after being rGO-based sensors, respectively. The samples’ pretreatment proce- transferred to the PET substrate. Clearly, even after the transfer, dures are described in detail in Section S1.2; DPV measurements were the rGO film preserves its highly exfoliated structure. performed by diluting the sample extract in PB (at the pH indicated in In order to evaluate the effect of the laser treatment on GO, the Electrochemical Measurements section). the EDX elemental analysis, Raman spectroscopy, and X-ray H2O2 determination was used to assess in-door disinfection fogging photoelectron spectroscopy were carried out. The EDX treatment residues. The treatment was performed according to EU elemental characterization of the film confirms a reduction of guidelines,32 and the decay of the H2O2 residue over time was evaluated onto a surface of 2 cm2 using a dedicated adhesive surface delimiter GO to rGO after laser treatment since the C amount in the (detailed description in Section S1.2). After fogging, 200 μL of PB (pH elemental composition EDX moved from 83.7% for the GO film 7.0) was placed in the surface delimiter for 1 min and directly used for (Figure S2) to 100% for the rGO (Figure 2A). The Raman the H2O2 residue quantification transferring 100 μL onto the Pt@rGO. spectra (Figure 2B) gave the classical Raman features for GO The same amperometric procedure described in the Electrochemical and rGO, consisting of a D band at ∼1350 cm−1, resulting from Measurements section was used for the measurement. The H2O2 the sp3 carbons from defects, the G band at ∼1580 cm−1, surface residues were evaluated immediately after the fogging treatment attributed to the sp2 carbons (refers to the stretching of the C−C and after 15 and 30 min. bonds), and the 2D band (∼2700 cm−1) for the rGO, that is ■ RESULTS AND DISCUSSION This work aims to propose a straightforward strategy to produce directly related to the structural organization in the two- dimensional plane of graphene derivatives.33 It is evident how the I(D)/I(G) ratio decreases after laser treatment, from 0.92 ± transferable nanostructured films to design effective lab-made 0.05 (GO film) to 0.64 ± 0.06 (rGO); this confirms the increase sensors for different applications. To achieve this goal, reduced in the −sp2 carbon domain compared to the −sp3. Besides, the graphene oxide (rGO) was chosen for the structural features and fitting of the rGO 2D band and the full width at half-maximum intrinsic conductivity, while the decoration with different MNPs value (∼76 cm−1) are compatible with randomly stacked single- was exploited to improve the performance and give additional layer graphene (around five layers ∼75 cm−1);36 thus, it is catalytic activities; controlled laser scribing was used as the film possible to affirm that the laser treatment (due to the high production technique. heating effect) promotes the expansion of the rGO sheets in the rGO Film Fabrication and Characterization. The laser- film, increasing their exfoliation.22,27,37 This is evident from the induced nanostructured film formation was carried out starting SEM micrographs for the GO (Figure S2) and rGO (Figure 2A) from a graphene oxide (GO) film incorporating noble metal films. XPS analysis of the GO film before (Figure 2C) and after cations (Mn+@GO), and a sketch of the MNPs@rGO sensor (Figure 2D) the treatment confirms the efficiency of the fabrication is shown in Figure 1. A detailed description of the chemical reduction of the GO.38 Before GO treatment, the C1s fabrication procedure is reported in the Laser-Assisted MNPs@ high-resolution spectrum highlights C−C carbon at a binding rGO Film Formation section. Furthermore, an overview video of energy (BE) of 284.2 eV and oxygen-containing functional the fabrication process can be seen in Video S1. groups at 286.3 and 288.1 eV, corresponding to hydroxyl/epoxy The laser treatment conditions were carefully optimized to carbon groups (C−O) and carbonyl groups (C�O), obtain a homogeneous and reproducible rGO film (see Section respectively. After the laser treatment, the C1s spectrum S2). Figure 2 reports the SEM images of the GO film before and demonstrates the GO reduction, with the carbon sharp peak after the laser treatment. The GO film shows a smooth and observed at 284.6 eV related to sp2-hybridized carbon (C�C), uniform structure with no evidence of lamellar nanostructures; and a substantial decrease of 61% for the C−O (286.7 eV) and of at this stage, the film is nonconductive. 10% for C�O (288.4 eV).39,40 This proves that highly After the laser treatment and press-transfer of the film onto exfoliated rGO can be produced starting from GO by laser the PET substrate (see the Sensors Fabrication section), the treatment.22,27 structure of the film appears wrinkled and is characterized by the MNPs@rGO Film Fabrication and Characterization. presence of highly exfoliated flakes with a nanometric section. The MNPs@rGO fabrication was carefully optimized for each 600 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article Figure 2. (A) SEM micrograph of the GO film after laser treatment. The inset reports a magnification of the rGO film and the % elemental composition. (B) Raman spectra of the GO (black line) and rGO (red line) film. (C) C 1s XPS spectrum of the GO film. (D) C 1s XPS spectrum of the rGO film obtained via laser treatment. All the analyses were performed after the film integration into the sensor. nanocomposite. GO and the metal ion (i.e., Au(III), Ag(I), and For the Au(III)@GO before the laser treatment, two solved Pt(II)) were co-filtered; then, the film was “one-shot” reduced components at BEs of 85.6 and 89.2 eV corresponding to the Au via CO2 laser-engraving and transferred onto the PET sensor 4f7/2 (Au3+) and Au 4f5/2 (Au1+) electron doublet, respectively, support. To obtain well-distributed MNPs on the rGO film, the are observed (Figure S4D).40,44,45 Despite the fact that the amount of GO, metal salt, and the number of laser cycles were cation used is the Au(III), the presence of Au(I) can be thoroughly optimized for each nanocomposite; a detailed attributed to the GO coordination ability through cation-π and description of the optimization is reported in Section S3 and electrostatic interactions.34,41,46−48 A slight presence of Table S1. zerovalent Au(0) was detected by XPS at 83.7 eV, which can Figure 3 shows SEM magnifications of the Au@rGO (Figure be attributed to the intrinsic chemistry of the GO rich in vacant 3A), Ag@rGO (Figure 3B), and Pt@rGO (Figure 3C) obtained electrons, electron doublets, and electronegative functional by employing the optimized conditions. The AuNPs and AgNPs groups potentially able to reduce the Au3+ to Au0, promoting the are round-shaped with a mean diameter of 9 ± 1 and 32 ± 2 nm, production of Au nanoseeds.42,47,48 After the laser (Figure 3G), respectively, whereas the Pt@rGO film shows cubic PtNPs with the AuNP formation is suggested by the BE shift toward lower a mean lateral size of 15 ± 1 nm (Figure 3D−F). The EDX values.44 The two sharp and asymmetric peaks recorded at 84.0 analysis confirmed the AuNPs’, AgNPs’, and PtNPs’ presence; and 87.7 eV, with a spin−orbit splitting value of 3.7 eV, are Figure S3 reports the elemental color mapping of the MNPs@ characteristic of the Au(0).23,49,50 rGO films, highlighting the AuNPs’, AgNPs’, and PtNPs’ Looking at Ag@rGO formation (Figure 3H), the Ag uniform distribution on the rGO film. The homogeneous MNP oxidation state passes from +1 (Ag(I)@GO) to 0 (Ag@rGO), distribution onto the rGO film can be attributed to the spatial and this is observable by the BE shifting to larger values: from distribution of the metal cations on the GO surface and the 364.8 to 368.4 eV for Ag3d5/2 and from 370.9 to 374.3 eV for uniformity of the laser treatment. In this context, Zhao and Ag3d3/2.40,49,51,52 The peak asymmetry and a separation value of Zhu41 demonstrated graphene’s ability to establish cationic-π 5.9 eV (Figure 3H) confirm the Ag presence in the metallic interactions along with the whole aromatic ring structure. The form.49,53,54 In the Ag@rGO, oxides of Ag are not observed, cations are further retained by the GO coordination chemistry indicating that the laser exclusively produced AgNPs.52 driven by the presence of epoxy groups on the GO surface and The Pt valence states on the GO (before laser) and rGO (after the oxygenated nucleophilic groups along the flake borders.41−43 laser) films were studied through the Pt 4f electron spectra. The GO reduction to rGO was also confirmed in the presence Before the laser treatment, the Pt 4f spectra have three peaks at of the metal salts for all the nanocomposites by XPS analysis 70.7, 73.2, and 75.8 eV (Figure S4F) assigned to Pt(0), Pt(II), (Figure S4A−C). The valence state of the Au, Ag, and Pt on the and Pt(IV), respectively.55,56 A very small fraction of Pt(0) is graphene bed was studied before and after the laser treatment; in observed in agreement with the chemisorption capability of Figure S4, the spectra of the Mn+@GO nanocomposites are GO.34 After the laser treatment (Figure 3I), the synthesis of reported, while Figure 3G−I shows the spectra after the PtNPs was confirmed by a positive shift of the Pt 4f peaks to BEs treatment (MNPs@rGO). of 71.4 eV (Pt 4f7/2) and 74.5 eV (Pt 4f5/2), with a spin−orbit 601 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article Figure 3. SEM micrographs of the (A) Au@rGO, (B) Ag@rGO, and (C) Pt@rGO films. The insets report a magnification of the formed MNPs. Size distribution of (D) AuNP, (E) AgNP, and (F) PtNP population onto the respective film. XPS spectra of the MNPs@rGO films formed with the laser treatment. (G) XPS spectrum of the Au 4f of the Au@rGO film. (H) XPS spectrum of the Ag 3d of the Ag@rGO film. (I) XPS spectrum of Pt 4f XPS spectra of the Pt@rGO films. All the analyses were performed after the film integration into the sensor. splitting value of 3.1 eV, which is close to the metallic value (Δ = 3.4 eV).57,58 Despite the fact that the Pt salt can form coordination complexes, the full reduction of the Pt precursor was confirmed by the Pt 4f7/2 binding energy (71.4 eV) quite similar to the bulk platinum one (71.2 eV).23,59 MNPs@rGO Formation Mechanism Proposal. A hy- pothesis about the laser-mediated MNPs@rGO formation is illustrated in Figure 4 and described below. The CO2-laser beam, in the near IR region (λ = 10.6 μm), induces a photothermal process able to remove the oxygen- Figure 4. Sketch of the proposed MNPs@rGO mechanism of nanostructured film formation. containing groups and lattice defects typical of the GO, driving the whole restoration of the sp2 domain24,60−62 proper of graphene structures25−28,63 and leading to the formation of rGO. consequent oxygen group elimination (in the form of O2, CO, We speculate that the MNPs@rGO formation is the result of an CO2, and H2O).60 The fast and violent expelling of oxygen- instantaneous cascade of events summarized as follows. The containing groups leads to the GO flake cracking and de- laser interacts with the GO film and drastically increases the local stacking; for this reason, the laser-treated zone physically temperature, inducing the atom thermal excitation with expands, becoming exfoliated.64,65 Simultaneously, the energy 602 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article Figure 5. (A) Cyclic voltammogram of the Au@rGO sensor in 0.5 M H2SO4. (B) Cyclic voltammogram of the Ag@rGO sensor in 0.1 M KCl. (C) Cyclic voltammogram of the Pt@rGO sensor in 0.5 M H2SO4. All the sensors’ CVs were performed at 100 mV s−1. (D) Peak-to-peak separation potential (ΔE) and (E) anodic peak intensity extrapolated from CVs performed in 3 mM Fe(CN)6 2−/3− in KCl 0.1 M at 25 mV s−1 using the proposed rGO and MNPs@rGO-based sensors. (F) Nyquist plots obtained from EIS analysis performed in 3 mM Fe(CN)64−/3− in 0.1 M KCl with the sensors integrating the rGO (●), Au@rGO (▲), Ag@rGO (×), and Pt@rGO (■) films. provided by the laser is sufficient to trigger the reduction of the protected from light (Figure S6). The electrochemical response metal source to the metal zero-valent state, where the electron retention for all the sensors over time (3 months) was studied transfer process is favored by the intimate cation-π interaction following the anodic peak intensity and the peak-to-peak between the GO and the metal cations.41−43 We suppose that separation using Fe(CN)62−/3− as the probe. After 3 months, a the photothermal energy produces electron excitation in the slight ΔE broadening was observed for all the sensors (100 ± 8, graphene, which, being rich in p-orbitals, is prone to host and/or 120 ± 8, and 147 ± 16 mV for Pt@rGO, Ag@rGO, and Au@ exchange electrons playing the role of electron donor to the rGO, respectively), while the peak intensity was quite stable metal cations.42,66 Then, the metal ions are first reduced to compared to the initial (anodic peak height retention ≥ 93%). metallic nuclei, which subsequently combine into MNPs up to The morphological nanostructures’ stability, both after reaching critical dimensions that result to be energetically consecutive measurement and after storage, was assessed by favored.41−43 The outcome is naked MNPs, uniformly SEM observations as well, and no significant variations were distributed and strongly anchored on the rGO substrate.62 observed, further confirming that the MNPs were strongly MNPs@rGO Sensor Electrochemical Characterization. anchored and stabilized by the rGO substrate (Figure S6). The integrated MNPs@rGO sensor fabrication is described in The electron transfer ability of the MNPs@rGO sensors was detail in the Laser-Assisted MNPs@rGO Film Formation investigated as compared to that of bare rGO employing inner- section, and the main steps are summarized in Figure 1. The and outer-sphere redox probes, that is, the ferro/ferricyanide electrochemical profile of the MNPs@rGO-based sensors was (Fe(CN)62−/3−) and the ruthenium couple (Ru(NH3)63+/2+). initially evaluated to highlight the crude chemistry of the The cyclic voltammograms obtained for both probes are nanocomposite; bare rGO is reported in Figure S5. As illustrated reported in Figure S7, and the extrapolated parameters are in Figure 5A−C, the presence of Au, Ag, and Pt nanostructures is listed in Table S2. For a better comparison of the sensors’ confirmed by the characteristic oxidation and/or reduction behavior, Figure 5D,E shows the graph of the peak-to-peak peaks observed;46,67 the latter is not observed for the bare rGO. separation (ΔE) and the anodic peak intensity obtained for the The obtained MNPs are electrochemically stable and strongly ferro/ferricyanide. anchored to the rGO; in fact, after 50 consecutive CVs The Fe(CN)62−/3− intimately interacts with the sensor performed in 0.5 M H2SO4, for Au@rGO and Pt@rGO, and 0.1 surface, and the electron transfer occurs directly via the redox M KCl for Ag@rGO, the characteristic metal peaks showed a probe adsorption on the used material;68 a quasi-reversible signal recovery of 99% (Au@rGO), 96% (Ag@rGO), and 88% behavior was obtained for all the sensors, and a significant ΔE (Pt@rGO) compared to the starting intensity (Figure S6). The decrease was observed from rGO (ΔE = 173 mV) to the MNPs@rGO stability was further assessed by simulating a MNPs@rGO. The best improvement was obtained with the discontinuous use of the sensors; consecutive independent CV Pt@rGO with a ΔE of 100 mV, immediately followed by Ag@ cycles (five scans each) have been run ten times; among rGO (ΔE = 117 mV) and Au@rGO (ΔE = 137 mV). The peak- measuring cycles, the electrode was dried under environmental to-peak separation, an index of the electron transfer ability, is conditions. Signal recoveries of 94% (Au@rGO), 91% (Ag@ consistent with the metal features expected (Pt > Ag > Au).34,35 rGO), and 83% (Pt@rGO) were obtained. The sensors’ long- As depicted in Figure 5E, the anodic peak intensity follows the term stability was eventually studied over 3 months (measuring opposite behavior of the ΔE, and for all the sensors, the anodic/ each week), keeping the sensors at room temperature and cathodic peak intensity ratio is close to 1. 603 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article Figure 6. (A) Cyclic voltammograms of 250 μM CA in PB + 0.1 M KCl (pH 7.0) performed with the rGO- (blue line) and Au@rGO (red line)-based sensors. (B) Cyclic voltammograms of 250 μM NO2− in PB + 0.1 M KCl (pH 4.0) performed with the rGO- (blue line) and Ag@rGO (red line)-based sensors. (C) Cyclic voltammograms of 250 μM H2O2 in PB + 0.1 M KCl (pH 7.0) performed with the rGO- (blue line) and Pt@rGO (red line)-based sensors. All the CVs were performed at 25 mV s−1; the black curve represents the blank measure performed with the used MNPs@rGO sensor. (D) DP voltammograms of CA from 0.5 to 100 μM at the Au@rGO sensor (in PB + 0.1 M KCl; pH 7.0). The black curve corresponds to the blank. The inset reports the calibration curve comparison between Au@rGO- (red line) and rGO (blue line)-based sensors. (E) DP voltammograms of NO2− from 1 to 100 μM at the Ag@rGO sensor (in PB + 0.1 M KCl; pH 4.0). The black curve corresponds to the blank. The inset reports the DP voltammograms of 60 μM NO2− at the Ag@rGO- (red curve) and rGO (blue curve)-based sensor. (F) Chronoamperograms obtained for the H2O2 from 5 to 2000 μM (in PB + 0.1 M KCl; pH 7.0). The black curve corresponds to the blank. The inset reports the magnification of the first six points tested (from 5 to 250 μM). To further explore the sensors’ electrochemical features, the the most suitable sensor was used. Before working with the electroactive surface area (ECSA) of the MNPs@rGO sensors analytes, the exploitable working potential window for each was calculated according to Randles−Sevick’s theory for quasi- sensor was explored in the related measuring buffer. The rGO-, reversible systems,69,70 and ECSAs of 8.20 ± 0.60, 10.59 ± 0.91, Au@rGO-, and Pt@rGO-based sensors were able to work in the 11.86 ± 1.96, and 11.85 ± 1.28 mm2 have been obtained for the −0.5 V/+1.0 V range, while the Ag@rGO was usable between rGO, Au@rGO, Ag@rGO, and Pt@rGO, respectively. +0.3 and + 1.0 V because of the presence of the typical Ag redox As expected, the CV profiles of Ru(NH3)63+/2+ (Figure S7B) peaks. In Figure 6A−C, the CVs that run in the presence of the evidenced a slight signal improvement for MNPs@rGO target analyte are shown, where the Au@rGO-, Ag@rGO-, and compared with the rGO, while a significant ΔE reduction was Pt@rGO-based sensors were employed to measure the CA observed (see Table S2). No relevant differences among MNPs (Figure 6A), NO2− (Figure 6B), and H2O2 (Figure 6C), were recorded since the ruthenium acts as an outer-sphere redox respectively; bare rGO was used as the control. probe, and for this reason, the different MNPs give similar CA cyclic voltammograms performed with the Au@rGO responses.68 sensor show a defined reversible behavior, and the obtained Finally, the resistance to the electron transfer was also studied peaks appear significantly improved compared with the bare with EIS; the Nyquist plot (Figure 5F) evidences a substantial rGO (Figure 6A). The same trend was observed for the NO2−, difference between the rGO- and the MNPs@rGO-based analyzed with the Ag@rGO sensor, where a greater oxidation sensors. Indeed, the rGO gives rise to the typical semicircle current than in bare rGO was observed (Figure 6B). The anodic with an Rct of 1.9 KΩ (calculated with the Randles equivalent peak shift to lower overpotential (ΔE ∼ 26 mV) suggests that circuit), while the MNPs@rGO appears in the Nyquist plot as the Ag enhances the electron transfer kinetics for the NO2− straight lines, clearly indicating the reduction of the electron oxidation.73 On the other hand, the Pt@rGO allows direct transfer resistance.65,71,72 analysis of H2O2 (Figure 6C); in this case, a large irreversible MNPs@rGO Sensor Application. After proving the cathodic peak at low negative potentials is obtained: this MNPs@rGO superior performance compared to conventional electrochemistry is typical of the H2O2 reduction catalyzed by Pt electrochemical probes, the sensors were used to detect various nanostructures.74 In this case, the bare rGO does not provide analytes of food, biological, and environmental interest to assess any response, since it is unable to assist the H2O2 reduction. their potentiality in real analysis. In particular, caffeic acid (CA), CA and NO2− were also analyzed using DPV (Figure 6D,E). nitrite (NO2−), and hydrogen peroxide (H2O2) were analyzed in In both cases, for increasing amounts of analyte, proportional, model solution and real samples/applications; for each analyte, reproducible (RSD ≤ 12.5%, n = 3), and defined anodic peak 604 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article currents were observed at an overpotential of 0.18 and 0.77 V, the working potential was studied by exploring the range from respectively. 0.2 down to −0.4 V; 0.0 V was selected as a compromise As shown in Figure 6D insets and in Figure S8, excellent between signal intensity and reproducibility. To maximize the linearity for the CA and NO2− was obtained in the range of 0.5− electrocatalytic performances, a pretreatment of the electrode at 100 μM (y/μA = 0.0351x/μM + 0.0298/μA; R2 = 0.999) and 1− +0.8 V (60 s) was applied before the measurement. This strategy 100 μM (y/μA = 0.0197x/μM + 0.0117/μA; R2 = 0.999), allows PtNPs to exert a higher catalytic activity since the respectively. presence of platinum oxides is disfavored and the oxide-free Pt The inclusion of Au- and AgNPs significantly improves the surface is more prone to assist the H2O2 reduction to H2O.74 electroanalytical performances of the rGO, enabling superior The cathodic current, which was employed as the analytical sensitivity (see the slopes of the calibration curves in Figure S8). signal, can be ascribed to the Pt surface regeneration after H2O2 An LOD of 50 and 120 nM was obtained for the CA and NO2−, adsorption and conversion to H2O. respectively; the LOD was calculated according to the following Figure 6F reports the chronoamperometric curves recorded formula: LOD = [(3σ)/m], where σ is the standard deviation of during the calibration; the Pt@rGO sensor gave reproducible the y-intercept and “m” is the slope of the linear regression. (RSD ≤ 10.5%, n = 3) cathodic currents proportional to the As proof of applicability, the Au@rGO and Ag@rGO sensors amount of H2O2 analyzed. On the other hand, no significant were used for CA-like compound identification in the coffee cathodic currents were observed for the bare rGO, definitely extract and NO2− quantification in fishing lake water. CA is a proving the nanozyme role of the Pt nanostructures. The sensor phenolic compound with a hydroxycinnamic structure charac- allows the determination of H2O2 between 5 and 2000 μM (y/ terized by an extreme antioxidant capacity, due to its ortho- μA = 0.0045x/μM − 0.1274/μA; R2 = 0.997), showing an LOD phenolic moiety. Coffee is a food rich in polyphenols�in of 0.6 μM; the calibration curve is shown in Figure S8. particular, CA and CA-like molecules75,76�for this reason, the Then, the Pt@rGO sensors were used to detect H2O2 surface quantification of these molecules can indicate the antioxidant residues after environmental fogging; the treatment was capacity of coffee. Before the CA quantification via the Au@rGO conducted following the EU and WHO recommendations on sensor, the phenolic profile of the coffee sample was COVID-19 prevention,32,78 and a detailed description of the characterized by liquid chromatography-tandem mass spec- treatment can be found in Section S1.2. In brief, the H2O2 trometry; o-diphenols, with a cinnamic structure, were found to residues’ decay onto a laboratory desk surface was evaluated be the most abundant phenols (∼134.3 mg caffeic acid equiv immediately after the fogging and after 15 and 30 min. Table S3 L−1). The coffee sample was treated before measurement, as reports the results obtained, and the decay of the H2O2 residue reported in Section S1.2, and the extract was properly diluted was observed to be consistent with the treatment time. To and measured with the Au@rGO sensor. To test the ability of demonstrate the sensor accuracy, the analyzed surface was also the sensor to properly quantify CA, the sample was fortified at spiked with three levels of H2O2 for each treatment time; the three CA levels. The data obtained are reported in Table S3: in very good recovery rate recorded (90−112%) proves the all the cases, the overpotential registered is close to 0.18 V sensor’s suitability for this application. Eventually, the Pt@rGO- (±0.01 V), confirming the presence of endogenous CA-like based sensor reusability was tested by measuring 10 consecutive compounds; good recoveries were obtained for all the times surfaces subjected to fogging treatment spiked with 60 μM fortifications (103−110%). H2O2; the data prove the reusability of the sensor, which did not In contrast, NO2− is an inorganic pollutant, toxic to human suffer passivation (RSD = 7.1%, n = 10). health, and widely used in agriculture as a fertilizer.77 The World Focusing on the literature of sensors based on laser-induced Health Organization (WHO) establishes 0.5 ppm (∼10.9 μM) graphene (LIG) and rGO, both pristine and decorated with as the maximum residual limit (MRL) in drinking water;73 from MNPs, the performance of the developed sensors is better or this arises the need to monitor this compound in the comparable (Table S4). For the three developed sensors and the environmental waters. Thus, we applied the Ag@rGO sensor relative applications, the LODs are lower than the majority of to the NO2− quantification in lake water samples. The water was the studies reported in the literature, demonstrating that the spiked at three NO2− concentration levels consistent with the proposed strategy returns effective sensing surfaces able, at the MRL (i.e., lower, equal, and higher); the unfortified sample did same time, to act also as transducers. The latter is a relevant issue not give any signal, excluding the presence of interferers. In all since rGO is commonly deposited on transducers/electrodes, cases, good recoveries were obtained (101−113%) (Table S3), while in the case of the LIG, the modified surface is often used and the recorded overpotential was close to 0.77 V (±0.02 V) in directly as the sensing layer. In this work, we have produced all measurements. transferable films that can be designed with the desired shape For both CA and NO2− analyses, the sensors provided and potentially transferred on different substrates. For the sake extremely reproducible results (RSDAu@rGO = 6.3% and of clarity, we want to underline that the performance of the Au@ RSDAg@rGO = 8.1%, n = 20), resulting in a reusable and not rGO was compared with sensors that have been employed for affected by fouling sensor surface. the determination of ortho-phenolic compounds; no paper in Resistance to passivation of the sensors was evaluated by 20 the literature reports the determination of CA with LIG or rGO- consecutive measurements of coffee and lake water samples based sensors. Overall, the obtained performance can be spiked with 7.5 μM CA and 40 μM NO2−, respectively. The ascribed to the presence of naked noble MNPs uniformly measurements were performed without washing the electrode distributed on a highly exfoliated rGO film; the rGO film ensures surface for consecutive DPVs. For both CA and NO2 − an excellent electron transfer ability, while the MNPs give determination, the sensors provided extremely reproducible additional electrocatalytic activities. As shown in Table S4, the results (RSDAu@rGO = 6.3% and RSDAg@rGO = 8.1%, n = 20), rGO and LIG decoration is often obtained using additional and resulting to be reusable and not affected by fouling. dedicated strategies (i.e., sputtering, electrodeposition, etc.); in Concerning the Pt@rGO sensor, to improve its sensitivity for this case, the decoration strategy does not need external MNP H2O2 detection, chronoamperometry was employed. Initially, synthesis, and the MNPs do not require capping agents, are 605 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article reproducible, and occur in few seconds concurrently to the rGO and hydrogen peroxide determination in samples; and formation, giving rise to transferable freestanding films. sensor performance comparison tables with respect to the ■ CONCLUSIONS Herein, a straightforward technique to produce freestanding literature (PDF) rGO-based conductive film patterning, fabrication, and transferring (AVI) films of rGO embedding naked-MNPs is proposed. The produced MNPs@rGO films are easily transferable onto flexible plastic supports to obtain transducers and functional sensing ■ AUTHOR INFORMATION Corresponding Authors surfaces, which result in enhanced stand-alone lab-made sensors. Ruslan Á lvarez-Diduk − Nanobioelectronics & Biosensors The nanocomposite-based films are obtained using a CO2 laser Group, Catalan Institute of Nanoscience and Nanotechnology plotter, which allows the controlled formation of hierarchic 2D/ (ICN2) CSIC and BIST, Bellaterra 08193 Barcelona, Spain; 0D structures with high resolution. This approach enables the orcid.org/0000-0002-9876-1574; Email: ruslan.alvarez@ formation of naked Au, Ag, and Pt regularly shaped icn2.cat nanostructures, uniformly distributed along with a highly Arben Merkoçi − Nanobioelectronics & Biosensors Group, exfoliated rGO film. As opposed to commonly used methods, Catalan Institute of Nanoscience and Nanotechnology (ICN2) our technique does not require multistep and cumbersome CSIC and BIST, Bellaterra 08193 Barcelona, Spain; ICREA procedures to form and decorate rGO-nanostructures and Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís capping agents and/or external synthesis is not needed to Companys 23, 08010 Barcelona, Spain; orcid.org/0000- generate MNPs, and the laser treatments drive the uniform 0003-2486-8085; Email: arben.merkoci@icn2.cat distribution of MNPs on the whole rGO film. We demonstrated the potentiality of MNPs@rGO-based Authors sensors for detecting different analytes in various samples and Annalisa Scroccarello − Nanobioelectronics & Biosensors thus for real-world applications. The sensors benefit from the Group, Catalan Institute of Nanoscience and Nanotechnology MNP features, which are enhanced by the highly exfoliated rGO (ICN2) CSIC and BIST, Bellaterra 08193 Barcelona, Spain; hosting substrate, also allowing us to obtain extra catalytic Department of Bioscience and Technology for Food, activity concerning the bare-rGO, as proved in the case of the Pt- Agriculture and Environment, University of Teramo, 64100 containing rGO nanocomposite. Teramo, Italy All in all, the proposed strategy is freestanding from the Flavio Della Pelle − Department of Bioscience and Technology hosting support, leading to the nanofilms’ ubiquitous transfer for Food, Agriculture and Environment, University of Teramo, onto different substrates. The produced MNPs are naked and 64100 Teramo, Italy; orcid.org/0000-0002-8877-7580 therefore are potentially modifiable with different molecules. Cecilia de Carvalho Castro e Silva − Nanobioelectronics & MNPs@rGO films will be further explored in a near future since Biosensors Group, Catalan Institute of Nanoscience and they could offer outstanding solutions and new opportunities in Nanotechnology (ICN2) CSIC and BIST, Bellaterra 08193 the (bio)sensoristic scenario and active substrates in different Barcelona, Spain; MackGraphe�Mackenzie Institute for optoelectronic devices (i.e., supercapacitor, field-effect tran- Research in Graphene and Nanotechnologies, Mackenzie sistor, SERS substrates, etc.) and applications (i.e., diagnostic, Presbyterian University, 01302-907 São Paulo, Brazil; bioimaging, etc.). orcid.org/0000-0003-3933-1838 ■ * ASSOCIATED CONTENT sı Supporting Information Andrea Idili − Nanobioelectronics & Biosensors Group, Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST, Bellaterra 08193 Barcelona, Spain; Department of The Supporting Information is available free of charge at Chemical Science and Technologies, Tor Vergata University of https://pubs.acs.org/doi/10.1021/acssensors.2c01782. Rome, 00133 Roma, Italy Claudio Parolo − Nanobioelectronics & Biosensors Group, Reagents and stock solutions; sample processing and Catalan Institute of Nanoscience and Nanotechnology (ICN2) fogging treatment; laser treatment optimization on the CSIC and BIST, Bellaterra 08193 Barcelona, Spain graphene oxide film; optimization of the MNPs@rGO Dario Compagnone − Department of Bioscience and film fabrication. SEM of the GO film before and after the Technology for Food, Agriculture and Environment, University laser scribing process; SEM micrograph of the GO film of Teramo, 64100 Teramo, Italy; orcid.org/0000-0001- before the laser scribing process; elemental mapping and 7849-8943 EDX analysis of Au@rGO, Ag@rGO, and Pt@rGO; XPS spectra of the Mn+@GO films before the laser scribing Complete contact information is available at: process; cyclic voltammogram of the rGO sensor in 0.1 M https://pubs.acs.org/10.1021/acssensors.2c01782 KCl; SEM micrographs and peak intensity obtained via CV to assess sensor stability and storability; cyclic Author Contributions # voltammograms of 3 mM Fe(CN)64−/3− in 0.1 M KCl A.S. and R.A.-D. contributed equally to this work. All authors and 3 mM Ru(NH3)63+/2+ in 0.1 M KCl performed with have given approval to the final version of the manuscript. the rGO, Au@rGO; Ag@rGO, and Pt@rGO sensors; Notes calibration curves of caffeic acid, sodium nitrite, and The authors declare no competing financial interest. hydrogen peroxide performed with Au@rGO, Ag@rGO, and Pt@rGO sensors; sensor components’ size; opti- mization of the laser parameters and nanoarchitecture ■ ACKNOWLEDGMENTS R.A.-D. acknowledges funding from the European Union precursors; electrochemical features of the rGO and Horizon 2020 Programme under grant no. 881603 (Graphene MNPs@rGO-based sensors; caffeic acid, sodium nitrite, Flagship Core 3). ICN2 is funded by CERCA Programme, 606 https://doi.org/10.1021/acssensors.2c01782 ACS Sens. 2023, 8, 598−609 ACS Sensors pubs.acs.org/acssensors Article Generalitat de Catalunya. Grant SEV-2017-0706 is funded by (14) Coroş, M.; Pogăcean, F.; Măgeruşan, L.; Socaci, C.; Pruneanu, S. MCIN/AEI/10.13039/501100011033. The authors also ac- A Brief Overview on Synthesis and Applications of Graphene and knowledge the project PID2021-124795NB-I00 funded by Graphene-Based Nanomaterials. Front. Mater. Sci. 2019, 13, 23−32. MCIN/AEI/10.13039/501100011033/and FEDER Una man- (15) Chua, C. K.; Pumera, M. Chemical Reduction of Graphene era de hacer Europa. F.D.P. and D.C. acknowledge the Ministry Oxide: A Synthetic Chemistry Viewpoint. Chem. Soc. Rev. 2014, 43, of Education, University and Research (MIUR) and European 291−312. (16) Zhang, Z.; Fraser, A.; Ye, S.; Merle, G.; Barralet, J. Top-down Social Fund (ESF) for the PON R&I 2014e2020 program, Bottom-up Graphene Synthesis. Nano Futures 2019, 3, 042003. action 1.2 0AIM: Attraction and International Mobility’ (17) Zhu, Q. L.; Xu, Q. Immobilization of Ultrafine Metal (AIM1894039-3). 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