Deep Eutectic Solvent-Assisted Synthesis of Ni-Graphene Composite Supported on Screen-Printed Electrode for Biogenic Amine Detection

1. Introduction

Deep eutectic solvents (DES) are formed by mixing two main components, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which interact by extensive hydrogen bonding to form an eutectic mixture with a melting point significantly lower than that of the individual components. These solvents can be formulated from food-grade materials such as organic acids, sugars, and other biocompatible HBDs, with choline chloride (ChCl), also known as vitamin B4, being one of the most widely used HBAs [1]. Initially classified within the broader category of ionic liquids, extensive research has provided a substantial amount of data to consider DESs as a specialized class of tailor-made solvents with many specific characteristics and unique effects [2]. The beneficial properties of DES, including sustainability, low toxicity, tunability, minimal vapor pressure, and the ability to dissolve a wide range of chemicals, have gained significant recognition in both scientific and industrial fields [3].

Due to the aforementioned advantages, DES has shown considerable potential as an effective medium for the production of a wide range of nanomaterials. This has been demonstrated in organic synthesis for the production of valuable organic compounds [4], as well as in the fabrication of inorganic nanomaterials and advanced carbon-based materials [5,6]. The successful synthesis of noble metal nanoparticles has been performed, with DES-mediated processes yielding materials with enhanced plasmonic and catalytic properties [7,8]. Beyond noble metals, the synthesis of transition metal sulfides, oxides, nitride, and phosphates has also been effectively carried out in DES, extending its applicability to a wider range of materials [5,9,10,11,12,13,14,15]. The most extensively studied technique for creating transition metal nanostructures in DES is electroplating. In this process, DES is used as an electrolyte medium, which not only enables the deposition of metal ions onto substrates, but also significantly influences the morphology and properties of the deposited layers [16,17]. Despite these advances, the preparation of pure transition metal nanoparticles in DES media beyond electrochemical conditions remains challenging.

Laser synthesis has emerged as a promising alternative approach for the synthesis of nanoparticles (NPs) in DES. In this method, the high temperatures generated in the focal spot of the laser beam are used to decompose the organic components of DES, resulting in the release of highly active species that facilitate the reduction of metal ions and the subsequent formation of metallic nanoparticles. This laser-induced pyrolysis can be considered to some extent analogous to the pyrolysis of metal-organic frameworks (MOFs), but offers a simpler and more streamlined synthetic pathway [4]. Previous studies using laser-induced pyrolysis of DES have primarily focused on continuous conductive metallic patterns on substrates [18]. Due to the need to achieve high temperatures within the laser beam, most research has focused on metallizing thermally stable substrates such as soda-lime glass [19,20,21]. Applications for the resulting metallic structures have been found in RFID (Radio Frequency Identification) tags, contacts for LED devices and, among others, in electrochemical non-enzymatic sensors [20].

A major advantage of laser-assisted synthesis of materials for electrochemical sensing is the ability to precisely shape electrodes through the localized nature of the laser process [22]. This localization eliminates the need for surfactants and surface binders such as Nafion, which are typically required to stabilize the NPs and to anchor them to electrode surfaces [23]. As a result, this method simplifies the manufacturing process and can lead to an increase in the purity of the synthesized nanomaterials. Significant potential for the detection of a wide range of analytes has been demonstrated by metal-based composites and advanced carbon materials synthesized using this technique [24], demonstrating the validity for further expansion of target analytes, including biogenic amines (BAs). Biosynthesized in microorganisms, plants and animals, BAs play a crucial role in various biological processes and are classified into endogenous and exogenous groups [25]. Monitoring of BA levels is essential in areas such as food safety and health care, as variations in their concentrations can lead to serious health consequences [26,27,28,29].

In this study, the feasibility of a one-step modification of commercially available screen-printed electrodes (SPE) via laser-induced synthesis in DES is explored. The initial DES composition was adapted from our previous works, which was aimed at synthesis of conductive continuous metal patterns [30,31]. The sensor activity of these composites towards a dopamine as a model biogenic amine is investigated. Nickel nanoparticles synthesized by this method exhibit excellent electrocatalytic activity in the electrooxidation of dopamine to dopamine quinone [32], while providing a cost-effective alternative to noble metals. Furthermore, the incorporation of graphene into the composite system is expected to improve the overall performance of the synthesized electrode by exploiting the synergistic effects of different nanomaterials [33]. The DES, composed of choline chloride (ChCl) and tartaric acid with dissolved nickel acetate and dispersed graphene, was drop-cast onto the surface of SPE and scribed using a 532 nm continuous wave laser. The fabrication scheme for Ni-G/SPE is shown in Figure 1. The experimental parameters were optimized using the Nelder-Mead method, which significantly reduced both time and consumable costs, thus supporting the sustainability of the research. The analytical characteristics of the sensor were evaluated using voltammetric techniques, including cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The ability of the sensor to detect dopamine over a broad linear range with high sensitivity and low detection limits is demonstrated by the study, proving the potential of laser-assisted DES synthesis in the development of advanced electrochemical sensors.

2. Materials and Methods

2.1. Materials

Nickel (II) acetate tetrahydrate (NiAc₂·4H₂O), Choline chloride (C₅H₁₄ClNO), Tartaric acid (C₄H₆O₆), sodium dihydrogen phosphate dihydrate (NaH₂PO₄*2H₂O), disodium hydrogen phosphate dodecahydrate (Na₂HPO₄*12H₂O), dopamine (C₈H₁₁NO₂) were purchased from Thermo Scientific (Fair Lawn, NJ, USA). Graphene suspension in water was purchased from XFNano (Nanjing City, CHINA). All reagents were analytical grade and used without further purification. Ultrapure Milli-Q water was used for all experiments (18.2 MΩ cm).

2.2. Synthesis of Deep Eutectic Solvent

Precisely measured amounts of choline chloride, an organic acid, and metal salt were combined to prepare deep eutectic solvent. The molar ratio of these components was maintained at 1:1:1. The graphene suspension was then added to the DES at different concentrations (3 - 50 wt%). The mixture was placed in sealed vials and heated at 110 °C until fully melted. The molten mixture was then transferred to a heated magnetic stirrer and stirred at 120–140 °C until a homogeneous, viscous liquid formed.

2.3. Electrode Fabrication

Electrodes were fabricated using a CW Nd:YAG laser with a wavelength of 532 nm and a maximum power output of 2 W. Commercially available SPEs (Poten Inc., Qingdao, China) served as conductive substrates for the application of the DES. A 5 μL volume of DES was uniformly applied to the working electrode of SPE using the direct transfer method. The modified electrodes were dried on a hot plate at 50 °C for 15 minutes. Subsequently, the electrode surface was scribed with a focused laser beam (NA = 0.4). The laser output power was varied between 100 and 1200 mW, while the scanning speed was maintained at a constant 1.4 mm/s. An XYZ mechanical stage controlled the substrate’s movement during the laser-writing process, ensuring precise patterning.

2.4. Nelder-Mead Algorithm

The Nelder-Mead algorithm was used to optimize laser power and graphene suspension concentration in DES. The primary performance metric for the efficiency of the electrode synthesis was its electrochemical response to dopamine. The optimization process began with the selection of three initial points in the parameter space, forming a triangular configuration (or simplex) (points 1–3 in Figure 2). The electrochemical response to dopamine was evaluated at each vertex, and the point exhibiting the highest response was identified. The algorithm iteratively performed a series of operations—contraction, expansion, and shrinkage of the triangle—to find the maxima of the electrochemical signal. The process continued until no further improvements in the response value were observed.

2.5. Characterization Techniques

The electrode’s morphology and elemental composition were examined via scanning electron microscopy (SEM) on a Zeiss Merlin system (Carl Zeiss, Oberkochen, Germany) and an INCAx-act energy dispersive X-ray spectrometer (EDX) (Oxford Instruments, Abingdon, UK). Characterization of the phase composition was carried out with X-ray diffraction (XRD) analysis using a Bruker D2 Phaser diffractometer with a LynxEye detector (Bruker-AXS, Karlsruhe, Germany). Raman spectroscopy was performed with a confocal Senterra spectrometer (Bruker, Billerica, MA, USA), utilizing a 532-nm solid-state laser at 10 mW power and a 50× objective, with spectra collected twice. X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250Xi electron spectrometer (ThermoFisher Scientific, Waltham, MA, USA).

2.6. Electrochemical Measurements

The electrochemical analysis was tested with a standard three-electrode cell using Corrtest CS300 potentiostat (Wuhan CorrTest co. LTD., Wuhan, China) at room temperature. Supporting electrolyte for determination of DA was phosphate buffer solution (0.1 M PBS, pH=7.0). SPE and Ni-G/SPE were employed as working, Ag/AgCl (3M KCl) as a reference and a platinum foil as a counter electrode. Additionally, small-volume sample analysis was performed using an SPE counter (carbon ink) and an SPE quasi-pseudo-reference electrode (AgQRE).

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used as electrochemical characterization techniques to assess the performance of the obtained sensors. CV measurements were conducted from -0.1 V to 0.8 V (vs. Ag/AgCl) with a scan rate of 50 mV/s. DPV curves were recorded with optimal parameters (amplitude 0.05 V, inc. E 0.004 V, pulse width 0.05 s, pulse period 0.5 s).

3. Results and discussion

3.1. Fabrication and Characterization of Ni-G/SPE via Laser-Assisted Synthesis Using the Nelder–Mead Optimization Approach

The Nelder-Mead [34] method was used to optimize the laser fabrication process by systematically adjusting the synthesis parameters to achieve a high sensor response of the fabricated electrodes to the target analyte. The Nelder-Mead method works by maintaining a simplex - a geometric figure consisting of n + 1 vertices in an n-dimensional parameter space. Through iterative processes of reflection, expansion, contraction, and shrinkage, the algorithm explores the parameter space to find the extremum of the objective function, which in this case was the maximization of the electrode response in 0.1 M PBS containing 100 µM DA.

The optimized parameters were laser power and graphene (G) suspension concentration in the DES. The laser power controls the temperature in the reaction zone at a given scanning speed, while the graphene suspension concentration affects the composition of the final structure. Therefore, these two parameters are essential for the one-step fabrication of the Ni-G composite on the SPE surface and allow to effectively tune the electrochemical properties of the fabricated electrodes. The feasible ranges for output laser power and graphene concentration were determined to be 500-1500 mW and 3-50 mass%, respectively. The parameter limits for laser power were chosen based on previous results for laser-assisted nickel patterning from DES [32], while the graphene concentration was chosen over a wide range to ensure comprehensive experimentation. The proposed approach, based on the Nelder-Mead algorithm, provides a means for interactive experimentation. Here, the functional properties of the material are monitored directly during the development of the synthesis methodology, in contrast to the trial-and-error approach, where the investigation of functional properties and the determination of optimal synthesis conditions often occur after obtaining a large number of samples under different conditions. Such an approach allows for a significant reduction in the number of required experiments for material optimization tailored to specific applications.

Based on a data on nickel pattern fabrication from DES [31,32], the starting point for Nelder-Mead optimization was chosen with a laser power of 540 mW and a graphene suspension concentration of 10 mass%. Two additional points, located at a small distance along each of the parameter axes, were determined to form the initial simplex. At each point, laser synthesis of the electrode was performed, followed by measurement of the electrochemical response to dopamine. Table S1 presents the values of laser power and graphene suspension concentration, along with the measured electrochemical signal for the simplex coordinates. SEM analysis of the samples produced at the simplex vertices (samples 1-8 on Figure 2 and sample 9 on Figure 3) revealed that structures fabricated from DES with low graphene concentration (up to 10%) exhibited a morphology typical of materials synthesized by laser-assisted deposition from DES [35], consisting of nickel nanoparticles forming a continuous conductive pattern. As the graphene concentration in DES increased, distinct nanoparticle-decorated graphene sheets became visible. The much larger surface area of graphene compared to bare SPE provides numerous interfacial nucleation sites for heterogeneous crystallization of nickel nanocrystals. This large surface area facilitates the uniform dispersion of nanoparticles and promotes the formation of numerous small nuclei rather than a few larger ones, resulting in efficient and uniform decoration. However, an excessive amount of graphene led to an inhomogeneous modification of the electrode surface, which compromised the sensor performance. As a result of optimization using the Nelder-Mead method, an optimal point with coordinates of 973 mW laser power and 19.7 mass% graphene suspension concentration in DES was obtained. The response of the electrode at this point 9 was significantly enhanced, reaching a current of 90.1 µA (Figure 2).

The electrode fabricated under the optimized conditions (sample 9) was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to investigate its morphological and structural properties. XRD patterns (Figure S1) show reflections corresponding to graphite from the original non-modificated screen-printed electrode itself [36], as well as a broad peak corresponding to the polyethylene terephthalate (PET) substrate of the SPE [37]. In addition, reflections of metallic nickel were observed, confirming the reduction of nickel ions and the formation of metal nanoparticles due to laser-assisted pyrolysis. The diffraction peaks for Ni at 2θ values of 44.5° and 51.8° correspond to the characteristic (111) and (200) planes, which are indicative of face-centered cubic (fcc) structures [38,39].

The elemental composition and chemical states of the Ni-graphene/SPE were thoroughly investigated by XPS. The survey spectra (Figure S2) show the presence of Ni, C, N and O. The high resolution XPS spectrum of Ni 2p (Figure 3d) can be deconvoluted into two spin-orbit doublets and two shake-up satellites, with the main peaks observed at 873.4 and 855.8 eV, corresponding to a spin-orbit separation of 17.6 eV. According to literature data, these spectral features are characteristic of nickel nanoparticles, which readily oxidize upon exposure to air, resulting in XPS spectra dominated by oxide species. The surface sensitive nature of XPS, combined with the propensity of transition metal nanoparticles to form surface oxide layers and adsorb impurities, typically results in the presence of Ni²⁺ peaks in the spectra [38,40]. XPS analysis of oxygen reveals two distinct peaks, located at binding energies of 531.3 eV and 532.8 eV. The peak at 531.3 eV is typically associated with OH-groups and nickel-oxygen bonds, indicating the presence of nickel oxide or surface hydroxide species [41]. This suggests that some nickel nanoparticles on the composite surface have undergone partial oxidation. The second peak at 532.8 eV corresponds to surface C-O bonds, which are characteristic of oxygen-containing functional groups on graphene sheets [42]. These groups may include epoxy or hydroxyl functionalities introduced during the laser-assisted synthesis process. The high-resolution XPS scan of N1s revealed the presence of three nitrogen species: the N–Ni peak centered at 399.6 eV [38,43], graphitic nitrogen at 401.4 eV [38,43], and a peak at 402.5 eV assigned to N2 trapped within the material during laser processing, as well as N-O surface contamination [44]. The formation of N–Ni and graphitic N species is highly favorable for electrochemical sensing applications and often results from high-temperature synthesis procedures [43,45]. The C 1s scan shows a spectrum typical of graphene structures and is consistent with the O 1s spectrum [46,47] The main peak at 284.6 eV can be attributed to the sp² carbon state, while the peaks at 286.3 eV and 288.9 eV indicate the presence of oxygen-containing functional groups.

The collected Raman spectra for the SPE, Ni-G/SPE, and graphene suspension all exhibit the characteristic D and G bands typical of carbon-based materials (Figure 3f) [48,49]. Additionally, the 2D band is observed, which consists of several components. The multi-component nature of the 2D band can be attributed to oxidized forms of multilayer graphene and graphite [50,51]. The as-received SPE shows peaks specific to partially oxidized, highly oriented graphite (Figure 3f). Considering the XRD data, the reasonable suggestion would be surface oxidation during manufacture and storage, while the main component of the SPE is graphite. As long as the width of this 2D band corresponds to layers non-uniformity, i.e., the presence of lattice disorder and defects, it can be concluded that laser irradiation promotes the formation of defects in the graphene layers. Therefore, laser heating not only initiates the pyrolysis of DES but also modifies the carbon component of the composite. The increase in defect sites and the formation of oxygen-containing groups can lead to a higher number of electrochemically active sites of graphene, thereby accelerating electron transfer during the electrochemical oxidation of dihydroxybenzenes, including catechol [52]. Since the target biogenic amine, dopamine, is a catecholamine compound and its electrooxidation involves the oxidation of hydroxyl groups (Figure S3), it is reasonable to assume that the formation of more defective graphene may also promote its electrooxidation.

3.2. Electrochemical Characterization of Ni-G/SPE and Dopamine Detection

Cyclic voltammetry (CV) measurements were performed in 0.1 M PBS supporting electrolyte in the presence of 50 µM DA to investigate the electrochemical behavior of the fabricated electrodes (Figure 4a). The Ni-G/SPE exhibited a significantly higher current response in CV compared to the bare SPE. This improvement can be attributed to the incorporation of graphene on electrode surface, resulting in a composite with a larger active surface area compared to unmodified SPE, which allows for a greater number of simultaneous electrochemical reactions per unit of geometric surface area. In addition, the decoration of graphene with Ni NPs leads to an increase in the electron transfer rate compared to the bare SPE, resulting in higher peak currents observed in CV. Faster electron transfer kinetics are also indicated by smaller potential differences between the anodic (Epa) and cathodic (Epc) peaks for the Ni-G/SPE. The smaller potential difference suggests that redox reactions on the modified electrode are more reversible. The Ni-G/SPE shows a well-defined oxidation peak corresponding to the oxidation of DA to dopamine quinone (DAQ) and a reduction peak corresponding to the reduction of DAQ back to DA (Figure S3).

Differential pulse voltammetry (DPV) was used for DA analysis because of its high sensitivity (Figure 4b). The anodic peak current (Ipa) dependence on DA concentration was linear in the range of 0.25 to 100 µM, indicating an analytical sensitivity of 0,91 µA µM^-1 (Figure 4c). The limit of detection (LOD) was determined to be 0.095 µM based on a S/N = 3, where S is the standard deviation of the blank response and N is the slope of the calibration curve. The analytical performance of the Ni-G/SPE for DA detection was listed along with sensors previously described in the literature [53,54,55,56,57,58,59,60]. The properties of the developed electrode are comparable with many existing analogues (Table S2). However, it is worth highlighting the advantages of the Ni-G/SPE over similar electrodes, such as the precise modification of the electrode with localized Ni-G deposition on the SPE surface controlled by laser beam movement, while keeping the fabrication procedure rather simple. In addition, the well-known advantages of SPEs, including cost effectiveness, versatility, scalability, and flexibility, make them indispensable tools in modern electrochemical research and commercial sensor development. The developed modification strategy for SPEs significantly improves their electroanalytical performance in DA sensing. Using DES, this methodology facilitates the effective incorporation of advanced composite materials on the electrode surface, resulting in improved sensitivity, linear range, and LOD. Moreover, this approach is highly versatile and can be extended to the detection of a wide range of analytes as well as to the fabrication of other advanced composite electrodes.

In addition, the ability of Ni-G/SPE to detect DA in small volume samples was demonstrated (Figure 4d). Aliquots of 50 µL containing three different concentrations of DA were used as model probes. The Ni-G/SPE electrode showed a prominent DPV response, and the peak current values fell within the linear regression, demonstrating the feasibility of the small volume sensing. However, the electrochemical response differs from data collected in bulk cells. In bulk electrochemical cells, mass transport is primarily driven by both convection and diffusion. This combination ensures a consistent and uniform supply of analytes to the electrode surface, maintaining a stable electrical double layer and consistent charge transfer kinetics. In contrast, in small volume probe measurements rely primarily on diffusion for mass transport because convection is minimal or absent [61]. The limited volume and potential for evaporation in droplets affects the peak currents and the shape of the DPV curves. In this respect, and considering the promising initial results, further in-depth investigations are needed in the future to optimize the analytical protocol.

Further studies were aimed at investigating the selectivity, reproducibility and repeatability of the analysis as well as testing the electrodes for DA detection on real samples. The repeatability of the experiment was evaluated by eight successive measurements in a solution containing 50 µM DA. The relative standard deviation (RSD) of the peak current was 3.3%, indicating excellent repeatability of the electrode. Reproducibility was investigated based on measurements with three independently fabricated electrodes, and the RSD was found to be 4.5%. Such favorable analytical performance of the electrode can be attributed not only to the electrode composition, but also to the optimized fabrication conditions that allow the modification of the SPE with Ni-G having good adhesion to the SPE surface. Adhesion was tested by performing a scotch tape test (Figure S5), confirming the robustness of the modified layer.

The most relevant coexisting substances in biological fluids are uric acid (UA) and ascorbic acid (AA), which are typically present at concentrations approximately 100 times higher than that of DA [62]. In this context, the selectivity of the fabricated sensor was tested by DPV in the presence of 50 µM DA and 100-fold excess concentrations of AA and UA, as well as other possible interferents and electrolytes. The results are presented in Table S3. All interferents tested, except UA, did not produce a significant analytical signal, and the signal deviation for DA did not exceed the calculated relative standard deviation for DA analyses without the interferents. For UA, an analytical signal was observed around 0.375 V. However, it does not affect the DA response, and the signal deviation for DA also did not exceed the RSD. The results obtained provide strong evidence for the excellent interference resistance of the developed electrode to the most abundant coexisting species.

The practical applicability of the Ni-G/SPE was confirmed by measurements on pharmaceutical samples of dopamine hydrochloride injections available at a local drugstore. The results are shown in Table S4. According to the manufacturer, the concentration of dopamine hydrochloride in the injection is 5 mg/mL (26 mM). Sample #1 was prepared by diluting a 9,6 µL aliquot of as-received injection solution in 50 mL 0.1 M PBS, suggesting the 5 µM of analyte in the final solution. Sample #2 was prepared similarly, except that the aliquot volume was increased to 38,4 µL. In addition, a protocol based on the standard addition method was also used (samples #3 and #4), where the dopamine hydrochloride injection solution in PBS was spiked with a DA solution of known concentration prepared in the laboratory. The developed sensor showed acceptable RSDs (N = 3) and recoveries, indicating satisfactory accuracy for real sample analyses.

4. Conclusions

This study demonstrates the feasibility and effectiveness of deep eutectic solvent-assisted laser synthesis for the fabrication of nickel-graphene-modified screen-printed electrodes for biogenic amine sensing. By using the Nelder-Mead optimization algorithm, key synthesis parameters-laser power and graphene concentration-were fine-tuned to significantly improve the electrochemical performance of the sensor for dopamine detection. The Ni–G/SPE demonstrated excellent analytical performance with a wide linear range from 0.25 to 100 µM of DA, a low detection limit of 0.095 µM, and 0,91 µA µM^-1 sensitivity. The practical applicability was confirmed by the successful detection of dopamine in pharmaceutical samples, achieving high accuracy and reliable recovery rates. The combination of laser-assisted synthesis and mathematical optimization was proven to be a powerful strategy for the rapid development of high-performance electrochemical sensors. The approach provides precise control over electrode modification, enables localization of material deposition, and eliminates the need for complex post-processing steps.