In Situ Laser-Induced Fabrication Of A Ruthenium-Based .
materialsArticleIn Situ Laser-Induced Fabrication of aRuthenium-Based Microelectrode for Non-EnzymaticDopamine SensingMaxim S. Panov 1 , Anastasiia E. Grishankina 1 , Daniil D. Stupin 2 , Alexey I. Lihachev 3 ,Vladimir N. Mironov 1,2 , Daniil M. Strashkov 2 , Evgeniia M. Khairullina 1 , Ilya I. Tumkin 1, *and Mikhail N. Ryazantsev 1,2, *123*Saint Petersburg State University, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia;m.s.panov@spbu.ru (M.S.P.); n.grshnk@gmail.com (A.E.G.); vova mironov 97@mail.ru (V.N.M.);e.khayrullina@spbu.ru (E.M.K.)Nanotechnology Research and Education Centre RAS, Saint Petersburg Academic University,8/3 Khlopina Street, 194021 St. Petersburg, Russia; stu87@yandex.ru (D.D.S.); danielstr@mail.ru (D.M.S.)Ioffe Institute, 26 Politekhnicheskaya, 194021 St. Petersburg, Russia; lihachev@mail.ioffe.ruCorrespondence: i.i.tumkin@spbu.ru (I.I.T.); mikhail.n.ryazantsev@gmail.com (M.N.R.)Received: 30 October 2020; Accepted: 23 November 2020; Published: 27 November 2020 Abstract: In this paper, we propose a fast and simple approach for the fabrication of theelectrocatalytically active ruthenium-containing microstructures using a laser-induced metaldeposition technique. The results of scanning electron microscopy and electrical impedancespectroscopy (EIS) demonstrate that the fabricated ruthenium-based microelectrode had a highlydeveloped surface composed of 10 µm pores and 10 nm zigzag cracks. The fabricated materialexhibited excellent electrochemical properties toward non-enzymatic dopamine sensing, includinghigh sensitivity (858.5 and 509.1 µA mM 1 cm 2 ), a low detection limit (0.13 and 0.15 µM), as well asgood selectivity and stability.Keywords: laser-induced metal deposition; ruthenium; dopamine; non-enzymatic sensors1. IntroductionThe fabrication of new materials for the detection of various biologically important analytes isof great interest in medical diagnostics, science, and industry [1,2]. Dopamine is one such analyte,along with glucose, hydrogen peroxide, amino acids, and many others [3]. Dopamine is the keycatecholamine neurotransmitter released by the brain and it plays a crucial role in the functioning ofseveral biological systems, including the central nervous system. An abnormal level of this biogenicamine in human blood is implicated in the development of a number of neurological diseases. Indeed,insufficient dopamine in the brain may lead to schizophrenia, Alzheimer’s disease, and Parkinson’sdisease, whereas high levels of dopamine result in Huntington’s disease [4]. Many methods can beused to determine the concentration of this and other disease markers in physiological fluids and modelsolutions, for example, high-performance liquid chromatography–mass spectroscopy (HPLC-MS) [5],fluorometry [6], colorimetry [7], and voltamperometry [8]. Despite their capability to detect lowconcentrations of an analyte, almost all of these techniques exhibit severe drawbacks: they areexpensive, time consuming, and can complicate experimental procedures. In turn, electrochemicalmethods are thought to be among the most effective methods of detecting low concentrations of manydisease markers, including dopamine, primarily due to their high sensitivity and fast response [3,9,10].As a rule, the electrochemically based techniques operate either in an enzymatic or in a non-enzymaticregimen [11]. Despite decent specificity, enzymatic sensing of dopamine has a number of limitations,Materials 2020, 13, 5385; ls
Materials 2020, 13, 53852 of 11such as low accuracy of detection, poor reproducibility and low stability as a result of enzymedecomposition (typically, tyrosinase) [12], and being subject to influence of the environment (pH andtemperature) [11]. In contrast, in the enzyme-free mode, direct detection of dopamine is ensured bycatalysis of ox-red reactions of this analyte occurring on the surface of an electrode [13]. In order toachieve the best effect in determining dopamine levels, an electrode must have a highly developedsurface area, ultimately allowing for a significant increase in sensitivity and stability, as well as reducedvalues of detection potentials. However, the problem of low selectivity associated with electrochemicalmethods remains unsolved [14].Numerous approaches can be used to manufacture materials with surface areas that demonstrate ahigh degree of porosity. Among them are inkjet printing [15], direct laser writing (DLW) [16], selectivelaser sintering (SLS) [17], screen printing [18], and chemical vapor deposition (CVD) [19]. Despitetheir many merits, some of these methods have substantial shortcomings, such as expensive reagentsand equipment [15,18,19], as well as insufficient adhesion properties of the produced structures [17]and low deposition rates [19]. On the contrary, we propose a simple and inexpensive method thathas practically no such disadvantages. This method deals with laser-induced deposition of a metalfrom a solution on the surface of various dielectric substrates (LCLD) [20,21]. In general, the mainfeature of LCLD is that the reduction reaction of a metal and its subsequent deposition on the surfaceof glass, glass ceramics or other dielectric materials occurs in a local volume of a solution within thefocus of a laser beam. Accordingly, it is possible to synthesize metallic and bimetallic microstructuresof different phase composition having a highly developed surface area and, as a result, exhibiting highelectrocatalytic activity toward various analytes. Previously, we were able to fabricate sensor platformsbased on copper [22], nickel [23], gold [24], platinum [25], iridium [25], molybdenum [26], silver [27],and cobalt [28], which are appropriate for glucose, hydrogen peroxide, and alanine enzymelesssensing. In the current study, we manufactured a ruthenium-based microelectrode to detect dopamineconcentration. Materials containing ruthenium are widely known and are used as enzyme-freesensors [29–36]. For example, porous ruthenium oxide (RuO2 ) is used to catalyze glucose and hydrogenperoxide, as well as to measure pH [30]. This usefulness is due to the fact that RuO2 exhibits highsensitivity, good electrocatalytic activity, outstanding thermal stability, and high corrosion resistance.In addition, it was recently shown that ruthenium disulfide (RuS2 ) demonstrates high sensitivity withrespect to dopamine detection due to its great stability, electronic configuration nature, availability ofcatalytic active sites, and superb electrochemical redox characteristics [36]. Thus, as a main part ofthis work, we developed a sensor platform based on ruthenium microstructures with good selectivity,decent stability, and high sensitivity to the non-enzymatic determination of dopamine.2. Materials and Methods2.1. MaterialsThe laser-induced deposition of ruthenium-based microstructures on the surface of glasswas performed using a solution containing 3 mM of triruthenium dodecacarbonyl (Ru3 (CO)12 )in N,N-dimethylformamide (DMF). These reagents were analytically graded and were purchased fromSigma Aldrich (St. Louis, MO, USA) for further usage without any additional purification.2.2. Synthesis of Ru-Based MicroelectrodeThe principal scheme of the synthesis setup is shown in Figure 1. A diode-pumpedcontinuous-wave solid-state Nd:YAG laser (Changchun, China) operating at 532 nm was used as a lightsource for thermally induced reduction and deposition of Ru microstructures. The laser output traveledthrough two aluminum mirrors and an optical separation cube, and was then focused on the sample(solution) using a standard microscope objective with a focal length of 15 mm. The solution containinga ruthenium(VI) carbonyl complex was placed in a special experimental cell, which was moved by acomputer-controlled XYZ-motorized platform. Further, part of the laser output was reflected back by
Materials 2020, 13, 5385Materials 2020, 13, x FOR PEER REVIEW3 of 113 of 12the cell andwasredirectedby an opticalseparationcube towardweb-cameraforthereflectedcell andbackwas byredirectedby anopticalseparationcube towarda web-cameraforainsitu nprocess.Here,theneutral-density(ND,fractionalof the laser metal deposition process. Here, the neutral-density (ND, fractional transmittance 25%)transmittance 25%) filter was inserted into an optical path in order to prevent optical damage to thefilterwas inserted into an optical path in order to prevent optical damage to the camera by an excess ofcamera by an excess of the 532 nm light. Finally, ruthenium microstructures were produced bythe 532 nm light. Finally, ruthenium microstructures were produced by scanning a laser beam focusedscanning a laser beam focused on the solution–glass interface along the vertical direction of the cellon the solution–glass interface along the vertical direction of the cell movement. Because of such lasermovement. Because of such laser writing, we were able to synthesize a ruthenium microelectrodewriting, we were able to synthesize a ruthenium microelectrode with a length of 10 mm and a widthwith a length of 10 mm and a width of 100 μm at a laser power of 1400 mW and a scanning speedof 100 µm at 1 a laser power of 1400 mW and a scanning speed of 7.5 µm s 1 .of 7.5 μm s .Figure 1. The schematic illustration of the experimental setup for the fabrication of the Ru-basedFigure 1. The schematic illustration of the experimental setup for the fabrication of the Ru-basedmicroelectrode: (1) diode-pumped continuous-wave solid-state Nd:YAG 532 nm laser; (2) aluminummicroelectrode: (1) diode-pumped continuous-wave solid-state Nd:YAG 532 nm laser; (2) aluminummirror; (3) optical separation cube; (4) microscope objective; (5) ND filter; (6) lens; (7) web-camera; (8)mirror; (3) optical separation cube; (4) microscope objective; (5) ND filter; (6) lens; (7) web-camera;computer-controlled XYZ motorized stage; (9) personal computer (PC); (10) experimental cell; (11)(8) computer-controlled XYZ motorized stage; (9) personal computer (PC); (10) experimental cell;fabricated Ru-microelectrode; (12) 3 mM Ru3(CO)12 in DMF; (13) glass substrate.(11) fabricated Ru-microelectrode; (12) 3 mM Ru3 (CO)12 in DMF; (13) glass Identificationofof Ru-BasedRu-Based based microelectrodemicroelectrode wasa ingusinga )coupledwithanenergy-dispersiveanalyzerelectron microscope JSM-7001F (SEM, JEOL, Japan) coupled with an energy-dispersive haracterizeitsatomiccomposition.PentaFETx (Oxford Instruments, UK) to characterize its atomic composition.The X-ray diffraction analysis (XRD) for phase identification of the synthesized rutheniumThe X-ray diffraction analysis (XRD) for phase identification of the synthesized ruthenium materialmaterial was performed on a Bruker D2 Phaser diffractometer equipped with a LynxEye detectorwas performed on a Bruker D2 Phaser diffractometer equipped with a LynxEye detector (Bruker-AXS,(Bruker-AXS, Karlsruhe, Germany) using CuKα (0.1542 nm) radiation in the 2θ angle range of 0 –Karlsruhe, Germany) using CuKα (0.1542 nm) radiation in the 2θ angle range of 0 –100 .100 .2.4. Impedance Measurements2.4. Impedance MeasurementsFor obtaining impedance spectra using high-speed and high-resolution EISFor obtaining impedance spectra using high-speed and high-resolution EIS methods [37]—AFmethods [37]—AF-EIS [38] and Fourier-EIS [39]—a homemade setup was used [38]. The measurementsEIS [38] and Fourier-EIS [39]—a homemade setup was used [38]. The measurements were providedwereprovided with 15 mV sweep-shape excitation voltage in the frequency range of 100 Hz to 40 kHzwith 15 mV sweep-shape excitation voltage in the frequency range of 100 Hz to 40 kHz with a 2 Hzwitha2 Hz resolution.To createthe electrochemicalcell, a ruthenium-basedmicroelectrodeand a Ptresolution.To create theelectrochemicalcell, a ruthenium-basedmicroelectrodeand a Pt referencereferenceelectrodewithsurfacea large areasurfaceareawere embeddedglass containingNaCl(Biolot,solutionelectrodewith a largewereembeddedinto glassintocontaining0.9% St. Petersburg, Russia). The impedance spectra approximation by the complex non-linear smadein equest).Figure2 2demonstratesthe CPEis eme usedused for theCPEis pedanceimpedanceofofwhichwhich equals:equals:Z 1W (iω)α(1)
Materials 2020, 13, x FOR PEER REVIEW4 of 121Materials 2020, 13, 5385 (4 of 11)(1)where α is the non-ideality parameter and W is the pseudo-capacitance with dimension S sα . Typically,whereα is thedescribenon-idealityparameterand InWparticular,is the pseudo-capacitancedimensionS sα.CPE elementsnon-idealcapacitors.α 0.5 can refer towiththe interfacebetweenTypically,non-idealcapacitors.In surfaceparticular,α 0.5Tocanrefer rodedescribewith ace[42–44].Toaccountfor thebetween excitation voltage and current response measurements by ADC (analog-to-digital converter),delaybetween texcitationvoltageinandthe parameterwas introducedthe currentmodel asresponsefollows: measurements by ADC (analog-to-digitalconverter), the parameter Δt was introduced in the model as follows:(2)Ym Ys eiω t (2) where roximation,the admittance(Figure2),s is admittancewherewasusedforforCNLSapproximation,Ys isYthe(Figure2), epeated10timesinordertoobtainstatistics.ω is the angular frequency. The measurements were repeated 10 times in order to obtain statistics.FigureFigure 2. TheThe equivalentequivalent schemescheme forfor describingdescribing the ruthenium microelectrode. Here, L 33 mHmH isis thetheparasitic inductance caused by the finite-time response of the ammeter.ammeter.2.5. ElectrochemicalElectrochemical StudiesStudies2.5.The of offabricatedRu-basedmicrostructureswere ere ric methods. All measurements were carried out on an Elins P30I potentiostat(Electrochemical InstrumentsInstruments Ltd.,Ltd., Chernogolovka,Chernogolovka, Russia)Russia) atat anan ambientambient temperaturetemperature inin aa standardstandard(Electrochemicalthree-electrode cell,cell, inin whichwhich platinumplatinum wire,wire, anan Ag/AgClAg/AgCl electrode,electrode, andand aa rutheniumruthenium vely.Cyclicvoltammetricstudieswerewere used as counter, reference, and working electrodes, respectively. Cyclic voltammetricstudies 1run atruna scan50 mVs mVbetween 0.9 andV 0.9vs. VAg/AgCl.Amperometricresponseswerewereat aratescanofrateof 50s 1 between 0.90.9andvs. 0.1MNaOH)withwere recorded by adding dopamine of various concentrations to the background solution rea,andhydrogenperoxidewereusedasinterferingNaOH) with simultaneous stirring. D-glucose, ascorbic acid, urea, and hydrogen peroxide were usedcomponentsdeterminingselectivity theof theRu-basedtoward ningselectivityofmicroelectrodethe Ru-based microelectrodetowarddopamine.3. Results and Discussion3. Resultsand DiscussionThe conductiveruthenium microstructures were fabricated by means of LCLD after optimizationof the experimental conditions, i.e., at a laser power of 1400 mW, at a scanning speed of 7.5 µm s 1 ,The conductive ruthenium microstructures were fabricated by means of LCLD afterand using 3 mM Ru3 (CO)12 in DMF. It should be noted that we were able to produce metal structuresoptimization of the experimental conditions, i.e., at a laser power of 1400 mW, at a scanning speed ofat a significantly higher scanning speed compared with other sensor-active materials previously7.5 μm s 1, and using 3 mM Ru3(CO)12 in DMF. It should be notedthat we were able to produce metalsynthesized using LCLD (3 times faster, 0.75 vs. 0.25 µm s 1 ). We did not expect photochemicalstructures at a significantly higher scanning speed compared with other sensor-active materialsreactions to contribute to the deposition process because the DMF solution of the ruthenium carbonylpreviously synthesized using LCLD (3 times faster, 0.75 vs. 0.25 μm s 1). We did not expectcomplex used in this work was transparent to the 532 nm laser light.photochemical reactions to contribute to the deposition process because the DMF solution of theThe results of the Ru-based microelectrode surface analysis using scanning electron microscopyruthenium carbonyl complex used in this work was transparent to the 532 nm laser light.(SEM) and energy-dispersive X-ray spectroscopy (EDX) are presented in Figure 3. Here, one can seeThe results of the Ru-based microelectrode surface analysis using scanning electron microscopythat the electrode had a non-flat, complex surface with two levels of development; specifically, it had(SEM) and energy-dispersive X-ray spectroscopy (EDX) are presented in Figure 3. Here, one can seelarge-scale 10 µm pores (Figure 3a,b) and small-scale 10 nm surface irregularity (Figure 3c). Accordingthat the electrode had a non-flat, complex surface with two levels of development; specifically, it hadto EDX data, the manufactured electrode was mainly composed of ruthenium and partially of oxygenlarge-scale 10 μm pores (Figure 3a,b) and small-scale 10 nm surface irregularity (Figure 3c).(Figure 3d). The peaks at 0.26, 1.06, and 1.74 keV corresponded to carbon, sodium, and silicon,respectively, the presence of which can be attributed to the substrate material (glass). These findings
Materials 2020,2020, 13,13, xx FORFOR PEERPEER REVIEWMaterials55 ofof 1212Accordingto5385EDX data, the manufactured electrode wasMaterials2020, 13,to5 of 11AccordingEDXwas mainlymainly composedcomposed ofof rutheniumruthenium andandpartially ofof oxygenoxygen (Figure 3d). The peaks at 0.26, 1.06, and 1.74partially1.74 keVkeV odium,and silicon,silicon, respectively,respectively, the presence of which can be attributedandattributed toto thethe substratesubstrate materialmaterial (glass).(glass).weresupportedthe supportedX-ray diffractionThe -ray esefindingswereby the Figurethatthefabricated rutheniummicrostructurescontainedboth metallicand oxide(RuOIn turn,2 ) ainedbothmetallicand4a)demonstratedthat the fabricatedmicrostructurescontainedbothmetallicand oxideoxidethe(RuOpresenceof rutheniummaypossibly explainrelativelyvaluesof the electrical(RuOphases.In turn, thedioxidepresenceof rutheniumdioxidethemaypossiblyexplainthehigh22)) phases.Inmaypossiblyhighexplainthe ode( rode( 1.2kΩ)anditssemiconductornature.values of the electrical( 1.2 kΩ) and its semiconductor nature.studiesare requiredfor a betterunderstanding.However,more detailedHowever,morestudiesare required for a better understanding.understanding.Figure3.TheThe SEM imagesimages (a–c)andEDXspectrum(d) (d)of ndEDXEDXspectrumof fabricatedthe fabricatedRu-basedmicroelectrode.FigureTheSEMSEM images (a–c)spectrum(d) of to consistof rutheniumAccordingto consistof rutheniumwith withweightAccordingto consistof (wt.%)of29.weight percentage (wt.%) of 29.Figure 4. (a) The XRD pattern of the Ru-based microstructures deposited on glass; (b) The admittanceFigureTheXRDXRDpatternpatternofofthethe Ru-basedRu-based microstructuresmicrostructures depositedonTheFigure4. nceadmittancespectrumofthe Ru electrodeobtainedusing the AF-EIS method;(c) The admittancespectrumof ;(c)TheadmittanceRu electrode obtained using the Fourier-EIS method. For both methods, the black squares ctrodeobtained usingthe Fourier-EISmethod.For boththe blacksquares correspondto the experimentalvalue,whereas thered circlesrefer methods,to the CNLSapproximation.Both the tothe experimentalwhereasredrefercirclesreferto theCNLS approximation.Both thethetoexperimentalvalue, value,whereasthe red thecirclesto theCNLSapproximation.Both the experimentaltechniques provide low-noise data, which can be perfectly fitted using the scheme illustrated in Figure 2.We evaluated the porosity of the resulting ruthenium electrode using impedance spectroscopyas the most important criterion for its further application as an enzyme-free sensor. The obtained
Materials 2020, 13, 53856 of 11spectra and the approximated elements of the equivalent scheme are shown in Figure 4b,c and Tables 1and 2, respectively. From Figure 4b,c, it is clear that the three-branch scheme in Figure 2 (CPE0 ,R1 -CPE1 , R2 -CPE2 ) gave a perfectly fitting result for both AF-EIS and Fourier-EIS. We consideredevery R-CPE branch in the scheme illustrated in Figure 2. First, we observed that the CPE0 branchhas α0 1. Moreover, the value of the W 0 was close to those of the capacity of the wires used as thecontacts with the sample. Thus, the CPE0 branch corresponded to the parasitic capacity leakage inthe wires. Second, the α-values of the R1 -CPE1 and R2 -CPE2 branches were significantly lower thanunity. This observation indicated that the surface of the ruthenium electrode consisted of two phaseswith different degrees of porosity—something confirmed by the SEM images in Figure 3a–c, in whichone can see two types of structures on the surface of the Ru electrode: 10 µm scale pores and 10 nmscale zigzag cracks. Therefore, the equivalent scheme in Figure 2 (except parasitic inductance and theCPE0 branch) could be directly associated with the Ru electrode’s surface morphology. In anotherwords, the electrical properties of the electrode material were in agreement with the properties of itssurface morphology. Indeed, α1 of the R1 -CPE1 branch corresponded to the more developed part of theelectrode surface, whereas the value of α2 obtained from R2 -CPE2 was associated with those areas thathave a lower degree of surface development. Furthermore, both the R1 -CPE1 and R2 -CPE2 branchesprovided an equal contribution to admittance and thus took into account that these two branches wereimportant for Ru-based microelectrode characterization.Table 1. Approximation results for the admittance spectrum obtained using the AF-EIS model.ParameterValueRelative Error, %R1 , Ω4.4 1037W 1 , S sα11.09 10 65α10.6041ParameterValueRelative Error, %-W 0 , S sα04 10 1050α01.024ParameterValueRelative Error, %R2 , Ω2.7 1037W 2 , S sα21.1 10 727α20.703Table 2. Approximation results for the admittance spectrum obtained using the Fourier-EIS model.ParameterValueRelative Error, %R1 , Ω3.8 1036W 1 , S sα11.27 10 66α10.5901ParameterValueRelative Error, %-W 0 , S sα06 10 1050α00.995ParameterValueRelative Error, %R2 , Ω3.1 10310W 2 , S sα27 10 829α20.733We studied the electrochemical properties of the synthesized Ru electrode. Figure 5a showsthe cyclic voltammograms of the ruthenium microstructures in dopamine solutions of variousconcentrations. A typical cyclic voltammogram (CV) has pronounced anode and cathode peaks ofdopamine. Two regions of anodic oxidation can be distinguished as follows: the first range laybetween 0.14 and 0.12 V, whereas the second interval of oxidation potentials was between 0.13 and0.52 V. These regions can possibly be attributed to two electrocatalytic oxidation processes: Ru2 /Ru3 and Ru0 /Ru3 , respectively. Using the direct amperometry method, we obtained such importantelectrochemical parameters as the limit of detection and the sensitivity of the fabricated microelectrodeto enzyme-free dopamine sensing. Figure 5b illustrates a typical amperometric signal showing how the
Materials2020,13,13,5385Materials2020,x FOR PEER REVIEW7 of7 of12 11electrochemical parameters as the limit of detection and the sensitivity of the fabricatedsuccessiveadditionsof dopamineof differentconcentrationsa backgroundsolutionat a potentialmicroelectrodeto enzyme-freedopaminesensing.Figure 5b toillustratesa edopamineshowing how the successive additions of dopamine of different concentrations to a backgroundconcentrationin 0.33turn,Vlinearintervalsof such current.change forRu thatelectrodelay in 1–100andsolution at aincreased;potential ofchangedthe FaradayIt istheclearthe Faradaycurrent100–5000µM.Thedopaminedetection concentrationlimits (LOD) increased;of dopaminefor theruthenium-basedincreasedas thein turn,linearintervals of suchmicroelectrodechange for the wereRucalculatedLOD 3S/b,whereS is thestandarddeviationfromlinearity,whereasforb isthetheslope of theelectrodeaslayin 1–100and100–5000μM.The detectionlimits(LOD)of dopaminerutheniumbased microelectrodewere rangescalculatedLOD in 3S/b,whereis thethestandarddeviationlinearity,calibrationcurve (the linearare asshownFigure5c). SThus,calculatedLOD fromvaluesfor thesewhereasb iswerethe slopeof thecalibrationcurve (thelinearranges areshown sensitivitiesin Figure 5c).attributedThus, thetotwointervals0.13 and0.15µM, respectively.ThemaximumcalculatedcalculatedLOD valuesfor thesetwo intervalswere0.13and 0.15 μM,maximumtheselinear rangeswere 858.5and 509.1µA mM 1cm 2, respectively.It isrespectively.known that Thethe CVarea rangeswere858.5and509.1μAmMcm 2,consequently, the sensitivity are directly associated with the degree of development of the 1electroderespectively.It is knownthe CVlimitarea ectly associatedsurface.Therefore,the lowthatdetectionsensitivitytherevealedby desurface.Therefore,thelowdetectionlimitandcan be explained by the high porosity of this material. The recorded electrocatalytic parametershighof theby the rutheniumelectrodecanmaterialsbe explainedthe usedhigh porosityof thisenzymelessmaterial.Rusensitivityelectrode revealedwere comparedwith severalelectrodethatbywerefor dopamineThe recordedelectrocatalyticsensing[36,45–49](Table 3). parameters of the Ru electrode were compared with several electrodematerials that were used for dopamine enzymeless sensing [36,45–49] (Table gure5. entialofthe Ru electrode recorded in the presence of different concentrations of dopamine at etriccurrentonthedopamine0.33 V; (c) Linear dependence of the measured amperometric current on the dopamine concentrations;Theamperometricresponse of theamperometriccurrent to theconsecutiveadditionof 10 μM(d)concentrations;The response (d)of thecurrentto the consecutiveadditionof 10 µMdopamine(DA),dopamine (DA), 3 μM ascorbic acid (AA), 3 μM uric acid (UA), and 3 μM D-glucose (Glu) in a3 µM ascorbic acid (AA), 3 µM uric acid (UA), and 3 µM D-glucose (Glu) in a background solution ofbackground solution of 0.1 M NaOH.0.1 M NaOH.TableComparisonof the electrochemicalparametersof some electrodeused forWealso3.testedthe selectivityof the Ru-basedmicroelectrodeinmaterialsthe presenceofenzymea number offree dopamine detection.interfering substances, such as ascorbic acid (AA), urea (UA), and D-glucose (Glu). Figure 5c illustratesthat the most pronounced changeLinearin theRangeFaraday current was observedby theadditionSensitivity(μAmM 1 of dopamine toMaterial of ElectrodeReferencesLOD (μM) 2)cm(μM)the background solution as opposed to other tested analytes. This means that the fabricated electrode
Materials 2020, 13, 5385Materials 2020, 13, x FOR PEER REVIEW8 of 118 of 121–100 and 100–0.13 andmay have quiteinandthe 509.1model solutionsand inRu decent selectivity regarding dopamine detection both858.5This work50
material was performed on a Bruker D2 Phaser diffractometer equipped with a LynxEye detector (Bruker-AXS, Karlsruhe, Germany) using CuK (0.1542 nm) radiation in the 2 angle range of 0 100 . 2.4. Impedance Measurements For obtaining impedance spectra
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