Engineering electrochemical sensors using nanosecond laser treatment of thin gold film on ITO glass

Accepted Manuscript Engineering electrochemical sensors using nanosecond laser treatment of thin gold film on ITO glass Evaldas Stankevičius, Mantas Garliauskas, Lukas Laurinavičius, Romualdas Trusovas, Nikolai Tarasenko, Rasa Pauliukaitė PII:

S0013-4686(18)32681-1

DOI:

https://doi.org/10.1016/j.electacta.2018.11.197

Reference:

EA 33198

To appear in:

Electrochimica Acta

Received Date: 5 October 2018 Revised Date:

22 November 2018

Accepted Date: 28 November 2018

Please cite this article as: E. Stankevičius, M. Garliauskas, L. Laurinavičius, R. Trusovas, N. Tarasenko, R. Pauliukaitė, Engineering electrochemical sensors using nanosecond laser treatment of thin gold film on ITO glass, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2018.11.197. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Engineering Electrochemical Sensors Using Nanosecond

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Laser Treatment of Thin Gold Film on ITO Glass

Evaldas Stankevičius a,*, Mantas Garliauskas a, Lukas Laurinavičius a, Romualdas Trusovas a,

Center for Physical Sciences and Technology, Savanoriu Av. 231, LT-02300 Vilnius, Lithuania

b

Stepanov Institute of Physics of National Academy of Sciences of Belarus, Nezavisimosti Ave. 68,

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a

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Nikolai Tarasenko b and Rasa Pauliukaitė a

220072 Minsk, Belarus

*Corresponding Author.

E-mail address: [email protected]

Keywords: gold nanoparticles, nanosecond lasers, thin gold films, heat treatment, cyclic voltammetry, electrochemical impedance spectroscopy

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ACCEPTED MANUSCRIPT Abstract

Direct generation of gold nanoparticles on ITO glass using a nanosecond laser is presented and the electrochemical properties of the gold modified ITO electrodes for detection of the ascorbic acid are analyzed. Gold nanoparticles were generated by nanosecond laser pulse

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irradiation of thin, 3–30 nm thick, gold films. It was found that diameters and the number of generated nanoparticles per unit area strongly depends on the thickness of the gold film when it is less than 10 nm. Furthermore, experiments have shown that the influence of laser processing parameters (the laser pulse energy and pulse number) to the size, the distribution

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and the area density of generated gold nanoparticles on ITO glass is negligible. Characterization of the electrochemical properties of the gold modified ITO electrodes by nanosecond laser showed that the fabricated electrodes could be employed in electrochemical

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sensing. Therefore, the demonstrated generation of gold nanoparticles on ITO by using the nanosecond laser approach opens new opportunities for the development of highly sensitive

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and low-cost electrochemical sensors.

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ACCEPTED MANUSCRIPT 1. Introduction In electrochemistry, metal nanoparticles are widely utilized as a functional unit to modify electrode surfaces [1-6] in order to develop (bio)sensors and energy production cells [7, 8] as well as for environmental applications [9, 10]. Metal nanoparticles provide four main advantages over macroelectrodes for electroanalysis: 1) enhancement of mass transport, 2) catalysis, 3) high effective surface area, and 4) control over local microenvironment [11].

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Nanosized noble metal particles become important for various application fields including biosensors, nanosensors, nanodevices, catalysis, and nanoelectrochemistry [12-15]. Particular interests are focused on the synthesis and application of gold nanoparticles (Au NPs) in the electroanalytical and electrocatalytic field due to the ability to enhance the electrode

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conductivity of the electrode surface and facilitate the electron transfer, thus, improving the analytical selectivity and sensitivity [16].

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Indium tin oxide (ITO) films on glass or quartz substrates are often used as electrodes surface due to their prominent characteristics such as an excellent electrical conductivity and wide electrochemical working window [17, 18], as well as low-cost [16]. ITO electrode modified with Au NPs possesses a faster electron transfer rate and larger current response than the bare ITO electrode [19]. Therefore, such electrodes can be applied for the creation of highly sensitive and selective future sensors. The modification of ITO electrode with Au NPs faces

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with the problem of the deposition of colloidal nanoparticles on ITO coating. The immobilization of AuNPs by using binding molecules such as thiols [20] and silanes [21] is ideal for glass surfaces [22], however, this method does not allow to obtain a high coverage of Au NPs on the electrode surface in the case of ITO coatings. Furthermore, the electroactivity

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of the Au NPs films fabricated employing this approach is influenced by the contamination of various compounds, such as the reactants, surfactant and binder molecules, as well as these

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methods have the time-consuming, complex synthesis procedures. Other more efficient Au NPs modification of ITO electrodes methods as centrifugation [23], electrochemical deposition [19], seed-mediated growth process [24, 25] or thin film thermal treatment can be used. Thermal treatment of thin gold coatings on ITO glass is a one-step strategy which does not use colloidal solutions, and the generation of Au NPs occurs directly on the desired surface. Heat treatment of thin metal films can be performed using thermal annealing furnace [26] or nanosecond lasers [27, 28]. The main advantages of nanosecond laser processing are the submicron treatment accuracy, the low thermal impact to the substrate and surrounding areas, and the selective generation of nanoparticles on desired place and shape of the surface [27, 29]. 3

ACCEPTED MANUSCRIPT The gold nanoparticles exhibit electrochemical properties which make the modified electrodes very attractive as a base for biosensors fabrication [30, 31]. It was already demonstrated that Au NPs on ITO electrode can be used for sensitive and selective detection of Cu2+ ion [32], of arsenic(III) [33] or for the determination of albumin bovine serum–biotin [34], heat shock protein 70 [35], and immunoassay of C-peptide [36]. Furthermore, the modified ITO

amperometric

detection

combined

with

a

dual-channel

electrophoresis system [38].

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electrodes can be exploited as a simple DNA biosensing platform [37] or for in-channel configuration

microchip

In this work, the problem of attachment of AuNPs was solved by a one-step generation of

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gold nanoparticles directly on the ITO glass by using nanosecond laser pulses is investigated. The geometrical and optical properties of generated gold nanoparticles were analyzed. Detailed electrochemical characterization of this NPs was performed using cyclic

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voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For the first time, a potential application of these AuNPs was tested for response to ascorbate as a possible use for electrochemical sensing.

2. Experimental section 2.1. ITO electrode modification with Au nanoparticles

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ITO electrode modification with AuNPs was realized by employing the nanosecond laser Ekspla NL220 operating at 532 nm wavelength with 35 ns pulse duration and 500 Hz repetition rate. The focusing conditions for different experiments were diverse. Therefore, the diameter of the Gaussian beam on the surface was 142 µm and 315 µm (in the level of 1/e2).

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The pulse energy was varied from 40 µJ to 160 µJ. Irradiation was performed with the different number of laser pulses: 1, 10, 100, 1000. Seven different thicknesses of the gold film

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on ITO glass (3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, and 30 nm) were used for the generation of Au NPs.

2.2. Preparation of gold films on ITO glass Gold films were prepared using magnetron sputter Quorum Q150T with a deposition rate of 0.2 nm/s. The film thickness of gold films was controlled by varying the sputtering time from 15 s to 150 s. This corresponds from 3 nm to 30 nm gold film thickness. 2.3. Characterization of the modified ITO electrodes SEM characterization: SEM images were taken using a scanning electron microscope JEOL JSM-6490LV. The accelerating voltage was 10 kV. All samples were imaged directly without additional coating. The identification and the size measurements of the generated Au NPs 4

ACCEPTED MANUSCRIPT were performed by processing SEM images with ImageJ program. Optical properties measurements were carried out using optical microscope Nikon Eclipse LV100 and optical fiber spectrometer Avantes AvaSpec–ULS2048. The illumination lamp of the microscope was used as a white light source. The optical system of the microscope was operated for the collection of light to the spectrometer from a small area of a sample

normalized to the spectra of unmodified ITO electrodes.

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(~ 700 µm2). The measured optical density spectra of Au NPs modified ITO electrodes were

Characterization of the electrochemical properties of the modified ITO electrodes: Electrochemical characterization was performed with Ivium Technologies CompactStat

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potentiostat/galvanostat. AuNPs/ITO/Glass electrode was working electrode (WE), and Ag/AgCl and Pt wire served as reference (RE) and counter (CE) electrodes, respectively. Electrochemical properties of these electrodes were characterized in 0.1 M K2SO4 and 0.01 M

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H2SO4 electrolytes in the potential range between -0.2 V and 1.2 V vs. Ag/AgCl employing CV. Electrochemical impedance spectroscopy (EIS) was employed to analyze the capacitive and resistive behavior of the modified electrodes. The spectra were recorded at oxidation and reduction peak potentials as well as in the double layer region with potential perturbation of 10 mV in the same 0.1 M K2SO4. The frequency range was from 60 kHz to 0.1 Hz with 5

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points per decade.

Sensing capabilities of the Au modified ITO electrode were tested using ascorbic acid, which has relatively low oxidation potential [39] and it is an important antioxidant. The analysis was performed using cyclic voltammetry (LSV) between -0.2 V and 1.5 V vs. Ag/AgCl and linear

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sweep voltammetry (LSV) between 0 and 0.5 V vs. Ag/AgCl at 100 mV/s scan rate in the ascorbic acid concentration range of 0-150 µM.

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3. Results and Discussion 3.1. Generation of Au nanoparticles on ITO glass The generation of gold nanoparticles on ITO glass using the nanosecond laser irradiation of thin gold film is schematically illustrated in Fig. 1. A thin gold layer coated on ITO glass (Fig. 1(a)) is irradiated by the nanosecond laser beam (Fig. 1(b)). During the nanosecond laser irradiation, the thin gold layer melts and stays in the molten phase as the energy transfer from the electron sub-system to the lattice is slow in gold [40]. In this case, the equilibrium between hot electrons and lattice takes place within a time limit of up 50 ps [41, 42]. In the melted thin gold film, hydrodynamic instabilities occur (Fig. 1(c)) causing self-organization of material to the droplets on the ITO covered glass (Fig. 1(d)). When droplets cool down, 5

ACCEPTED MANUSCRIPT they form gold nanoparticles (Fig. 1(e)). The formation of gold nanoparticles using the nanosecond laser irradiation of the thin metallic coatings can be explained merely by the spinodal dewetting process [43-45]. The spinodal dewetting in the melted thin film occurs when attractive intermolecular forces exceed the stabilizing effect of interfacial tension [4547]. Then the thin film becomes unstable, the thermal fluctuations start to grow. The growth

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of thermal fluctuations leads to spontaneous destabilization of the thin film and formation of droplets/nanoparticles [48].

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3.2. Influence of the gold film thickness on the size, density and optical properties of generated Au nanoparticles Thin gold films with different thickness (3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, and 30 nm) were prepared on ITO glass in order to find out the impact of the gold film thickness on the size and density of generated Au NPs. The Au coatings were produced using

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magnetron sputter. The prepared films were irradiated by a single nanosecond laser pulse with the laser energy density of 505 mJ/cm2 (Fig. 2(a-c)). SEM images of generated Au NPs when the thickness of the gold coating on ITO glass is 3 nm, 5 nm, and 20 nm are shown in Fig. 2(d-f). The micrographs indicate that the size and density of generated Au NPs depend on the gold film thickness. Characterization of generated Au NPs was performed using analysis

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of SEM images with ImageJ software. The characterization method of generated Au NPs in detail is described in Supplementary materials. The size distribution of Au NP’s for all prepared thickness of Au coatings are given in Fig. 3(a-g). The obtained average diameter and density of generated Au NPs on ITO glass using the single nanosecond laser pulse for

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different thickness of Au films are presented in Fig. 3(h). Nanoparticles size histograms show that the smallest nanoparticles with the lowest size dispersion are generated using the thinnest gold film (3 nm). The average diameter of

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generated NPs, in this case, is 52±11 nm and the size range is 30-90 nm. For 5 nm gold film thickness, the average diameter of Au NPs is higher (90±28 nm) and distributed in the broader size range (30-160 nm). When the gold coating thickness is 10 nm, the average size of NPs increases up to ~200 nm, and Au NPs size distribution covers the range from 40 nm up to 450 nm. For thicker gold films (15-30 nm), the NPs distribution range expands from 40 nm up to 570 nm. The thicker gold film, the higher is size dispersion of the generated NPs. The results in Fig. 3 expose two maximums in the size distribution of NPs for thicker gold films (15-30 nm). The first maximum is in the range of 40-90 nm and the second maximum is in the range of 250-400 nm. The second maximum in the size distribution of NPs for thicker gold 6

ACCEPTED MANUSCRIPT films moves to the larger size of NPs. For 15 nm, 20 nm, 25 nm and 30 nm gold film the second maximum is ~ 250 nm, ~ 320 nm, ~ 360 nm, and ~ 400 nm, respectively. Despite that, the average size of generated NPs in case of thick gold films (15-30 nm) is similar and varies in the range of 223-235 nm as the increase of the second maximum in the NPs size distribution is compensated by a large number of small size nanoparticles (40-120 nm).

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The estimated density of nanoparticles (the average number of nanoparticles per square micrometer) is given in Fig. 3(h) (red dots). The highest density of NPs were obtained for the thinnest 3 nm and 5 nm Au coatings, ~ 40 µm-2, ~ 20 µm-2, respectively. The density of generated NPs using thicker Au coatings (10-30 nm) is much lower and varies in the range of

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2-4 µm-2. The experimental results demonstrate that the size, density and size dispersion of generated gold NPs strongly depend on the thin gold film thickness (3-5 nm). For thicker gold

and it changes only slightly.

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films (10-30 nm), the influence of the film thickness to the generated NPs parameters is weak,

Additionally, the optical properties of generated gold nanoparticles were measured. The optical density spectra of 3 nm and 5 nm gold coatings before the laser treatment are shown in Fig. 4(a). In this case, the measured spectra match to the dependence of the absorption coefficient on the wavelength [49], and it shows that the decrease of the passed light intensity

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through the different thickness coatings mainly is determined by the light absorption. The optical density spectra of 3 nm and 5 nm gold coatings treated by the single laser pulse with the laser energy density ~ 505 mJ/cm2 is presented in Fig. 4(b). The measured spectra have maximum values of optical density at 540 nm and 570 nm for 3 nm and 5 nm thickness gold

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films, respectively. These values fit the wavelength of the surface plasmon resonance, and the shift of the resonance for different gold film thickness indicates the generation of different size nanoparticles [50]. The surface plasmon resonance was not observed for nanoparticles

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generated from gold films thicker than 10 nm. This result can be determined due to several reasons. The first reason, the size distribution of generated NPs for thick coatings (≥ 10 nm) is wide (Fig. 3(c-g)). Therefore, the optical density spectra present the total resonance contribution of various size of NPs. The second reason, the average size of the produced nanoparticles is greater than 200 nm. Therefore, the absorption and scattering spectra of such large NPs is broad due to a higher order of plasmon modes (electrons do not have a uniform resonant frequency).

3.3. Influence of the laser pulse energy and number on the size and density of generated Au nanoparticles 7

ACCEPTED MANUSCRIPT The experiments were performed to find out the influence of the laser pulse energy and their number on the size and density of generated Au NPs. The first experiment was executed by irradiating 5 nm thick gold film with the single laser pulse when pulse energy was 40 µJ, 80 µJ, 120 µJ, 160 µJ. In the experiment, the laser beam diameter on the sample was ~ 315 µm. The size distribution of generated NPs is depicted in Fig. 5. The dependence of

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average diameter (black squares) and density (red circles) of generated Au NPs on pulse energy is shown in Fig. 6(a). The experimental results demonstrate that the size distribution, the average diameter, and density of generated nanoparticles are independent on the laser pulse energy and only the counts of NPs is different. In all cases, the NPs size distribution has a peak around 80 nm, and it meets the average value of generated NPs diameter. The

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measured density of generated NPs is ~ 27 µm-2. The difference in the counts of NPs arises due to the different size of the area from which NPs were counted as it is well known that the

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higher pulse energy affects, the larger area [51]. The decrease in NPs counts for 160 µJ energy pulse can be explained by the evaporation of the Au coating during the laser processing. Therefore, the use of high energy pulses is ineffective and even can damage ITO layer on glass. The damage threshold of the ITO layer with the coated thin gold film was measured using Liu proposed method [52, 53]. It was found that the threshold is

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~ 900 mJ/cm2 and it does not depend on the layer thickness of the coated gold film. The second experiment was carried out by irradiating the 5 nm thick gold film with a different number of laser pulses (1, 10, 100 and 1000) when the pulse energy was 80 µJ. The laser beam diameter on the sample was the same as in the first experiment (~ 315 µm). The size

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distribution of generated NPs is shown in Fig. 7. The dependence of average diameter (black squares) and density (red circles) of generated Au NPs on pulse number is presented in Fig. 6(b). In this case, the number of the laser pulse does not have any impact on the

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characterization parameters (size distribution, the average diameter, density, and counts) of generated NPs. The values of the average diameter and density of generated NPs are very similar to the previously discussed experimental results (average diameter ~ 80 nm, density ~ 27 µm-2). These results indicate that gold film melts and crystallizes cyclically with each laser pulse as it has enough time between laser pulses (2 ms) to cool down [54]. This cyclicality does not have any influence on the Au NPs generation. 3.4. Electrochemical characterization of laser-generated gold nanoparticles on ITO glass Based on the results presented above, two Au NPs modified ITO electrodes with a size of 25 x 25 mm2 were prepared. The decoration was performed by laser treatment of ITO glass 8

ACCEPTED MANUSCRIPT samples with a different thickness of thin gold films (5 nm and 20 nm). The illustration of ITO modification with Au NPs is depicted in Fig. 8. During the modification process was chosen 17.5 mm/s laser beam scanning speed in order to get 50 % overlap of laser beams. The laser energy density and pulse repetition rate were 214 mJ/cm2 and 500 Hz, respectively. The ITO decoration with Au NPs of 25 x 25 mm2 area took ~ 17 minutes. As described in

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previous sections, the average Au NPs size was ~ 90 nm and ~ 220 nm for 5 nm and 20 nm thick Au films, respectively. The obtained Au NPs on ITO covered glass were characterized electrochemically.

Obtained AuNPs on ITO glass were characterized electrochemically employing cyclic

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voltammetry (CV) using electrochemical cell (Fig. 8(b)) and electrochemical impedance spectroscopy (EIS). For comparison, control CVs were also recorded at bare ITO and bulk Au electrodes in 0.1 mol/L K2SO4 with 0.01 mol/L H2SO4 between -0.2 V and 1.2 V vs. Ag/AgCl

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at 50 mV/s. For Au characterization 0.5 mol/L H2SO4 is usually used as an electrolyte [55]; however, in the case of the thin films, Au is stripped from the electrode surface in such an acidic solution during measurements. Therefore, the less acidic solution was chosen by decreasing acid concentration, and sulfate was used to compensate for the loss of ionic strength and assure fast charge transport. It was determined that 0.1 mol/L K2SO4 is sufficient

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for these samples.

Fig. 9 demonstrates CV differences between the different electrodes. The highest current density was at the bare gold electrode (Fig. 9, green curve). ITO had no redox activity with capacitive current density about 0.4 mA/cm2 (pink curve) indicating that all changes in CV

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after electrode modification with AuNPs was coming from the nanoparticles. Higher current density was observed at AuNPs generated from the thinner gold film, i.e. 5 nm. This is most probably related to an electroactive area, which is proportional to the oxide

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reduction peak area at ca. 0.8 V [56]. As seen, the electrode with Au NPs obtained from the 5 nm thick gold film (Fig. 9, black line) has a more typical profile to Au surface [10]. The reduction or oxidation form maxima due to two main reasons in this case: 1) surface properties, i.e, when the whole surface is reduced or oxidized the current drops because these reactions slow down; 2) due to reaction rate differences at different potentials. The reduction wave at ~-0.17 V comes from AuNPs and appears due to gold deposition (1) while the peak at ~1.1 V is for gold oxidation in acidic solution (2) [45]: Au3+ + 3e- ⇄ Au

(1)

2Au +3H2O ⇄ Au2O3 + 6H+ + 6e-

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ACCEPTED MANUSCRIPT At ~0.8 V reduction of gold oxide takes place which is a backward reaction (1). The electrode covered with larger AuNPs (Fig. 9, blue line) had a different profile, and the Au redox peaks were less defined as in the case of the AuNPs obtained from the 5 nm thick layer. Moreover, the peak height in the case of smaller Au NPs (generated from 5 nm Au film) increased with the number of cycles until the 5th cycle and then reached steady state

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(Fig. S2(a)). However, the redox waves of the electrode with larger Au NPs (generated from 20 nm Au film) decreased with the number of cycles (Fig. S2(b)) until Au redox waves almost disappeared due to passivation of the gold surface. The increase of the reduction current at -0.1 V is typical for Au, where reaction (1) takes place in an acidic medium. The ill-

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defined redox peaks of larger Au NPs shows that gold, in this case, is passive and it might be caused by the lower density of nanoparticles on the ITO glass (Fig. 3(h)) since SEM investigation after the electrochemical measurements showed no changes in the Au NPs size

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or morphology.

EIS is a powerful tool to observe electrochemical processes occurring at the interface of the electrode and solution. EIS spectra were recorded at the reduction and oxidation peak potentials as well as in the double layer region. Fig. 10 presents Nyquist and Cole-Cole plots. The potentials for EIS registration were chosen according to the processes occurring at AuNP

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modified electrodes taken from CVs (Fig. 9): Au reduction at -0.2 V or 0.0 V; oxide reduction at -0.45 V and 0.80 V; and Au oxidation at 0.9 V and 1.10 V at AuNP modified electrodes formed from 5 nm and 20 nm Au films, respectively. Double layer region was also investigated by recording spectra at 0.05 V (5 nm) and 0.50 V (20 nm). As seen, EIS spectra

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at the bare ITO (Fig. 10(a)) are different from those at modified electrodes (Fig. 10(b,c)): ITO represents purely capacitive behavior in the potential region from -0.2 V to 0.9 V while AuNPs have more significant influence from the charge transfer during redox processes,

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especially Au reduction at AuNPs from 5 nm film (Fig. 10(b)). A good agreement with CV was observed that AuNPs generated from thinner Au film cause a decrease in impedance values.

In order to have a deeper insight into processes occurring at the interface of these electrodes and solution, fitting to equivalent electrical circuits was performed. In all cases, except ITO at 0.05 V, simplified Randles circuit was applied. It consisted of cell resistance (RΩ) in series with a parallel couple of a charge transfer resistance (Rct) and CPE as non-ideal double layer capacitance modeled as: CPE = -1/(Ciω)α

(1), 10

ACCEPTED MANUSCRIPT where the capacitance C describes the charge separation at the double layer interface and the α exponent is due to the heterogeneity of the surface [57]. The analysis results are presented in Table 1.

8.94

0.953

0.05

-

7.36

0.45

5336

5.80

0.90

3991

4.92

-0.20

6.09

0.05

160

0.45

434

13.9

0.872

0.90

43.7

10.4

0.881

0.00

422

5.35

0.884

0.50

737

3.88

0.894

0.80

688

3.30

0.897

1.10

548

2.87

0.901

0.958

0.960 0.958

11.0

0.868

9.47

0.872

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AuNP/ITO (20 nm)

4221

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AuNP/ITO (5 nm)

-0.20

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ITO

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Table 1. EIS parameters obtained by fitting to equivalent electrical circuit spectra presented in Fig. 10. CPEdl, µF cm-2 sα-1 α Electrode E, V Rct, kΩ cm2

According to the analysis results, RΩ was 92.9 Ω cm2 for ITO, 59.8 Ω cm2 for AuNPs from 5 nm film, and 69.7 Ω cm2 for AuNPs from 20 nm film. This fact showed that modification of

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ITO decreased the cell resistance. As seen from Table 1, ITO has a tremendous charge transfer resistance of MΩ cm2 while modification of the surface with AuNPs from 20 nm film

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decreased Rct at least ca. 10 times over whole potential region studied. In the case of AuNP/ITO from 5 nm film, similar Rct to other AuNPs was just in the double layer region and Au oxide reduction place, while it was still 10 times lower at Au oxidation and 100 times lower at Au deposition potentials. These data are in a good agreement with those from CV. Comparing the double layer capacitance changes between 3 electrodes the tendency was different and followed the sequence: AuNP/ITO (20 nm film) < ITO < AuNP/ITO (5 nm film). These data are also confirmed graphically presenting EIS spectra in Cole-Cole plots as complex capacitance (Fig. 9(d)). The lowest Rct and the most significant CPEdl values might also be related to the heterogeneity of the surface represented by the exponent α. α closer to 1

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3.5. Application of laser-generated gold nanoparticles on ITO glass to electrochemical sensing These electrodes were also tested for L-ascorbate detection. L-Ascorbic acid in a strongly acidic solution is electrooxidized to diketogulonic acid (Fig. S3). Furthermore, L-

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ascorbic acid, especially on metal electrodes, is oxidized to unstable dehydro-L-ascorbic acid. In acidic medium, electrooxidation occurs without strong adsorption of ascorbic acid on the electrode. However, its adsorption is the limiting step of the electrochemical reaction in

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neutral and alkaline media [58]. Although in the latter cases oxidation currents are sometimes higher (depending on the electrode) as well as peak potential is lower [59-61], which makes an easier determination of this compound with lower interferences. The CVs obtained at both electrodes (with different AuNPs size of 90 nm and 220 nm) at different ascorbic acid additions are presented in Fig. 11. As seen from CVs, the current increase proportional to the

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ascorbate concentration is visible at ~0.25 V vs Ag/AgCl. The shape of the response is typical to electrochemical process, where at the beginning signal increases due to the concentration of ascorbate at the electrode surface. Later on, the diffusion of ascorbate ions take longer and therefore the signal either drops slightly (Fig. 11(a,c)) or remains in plateo (Fig. 11(b,d)).

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Such an electrode modification shows electro-catalytic activity because usually, ascorbate oxidation takes place at 0.35-0.45 V depending on the electrode and electrolyte. Interestingly, that ascorbate influences also oxidation process of gold and the peak at ~1.1 V vs. Ag/AgCl

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(Fig. 11(a)) increases with increase in the ascorbate concentration. This means that the product of ascorbate oxidation influences kinetics probably due to the electron transfer process following an adsorptive mechanism [10]. Unfortunately, when the electrode potential is swept to Au oxidation (1.2 V vs. Ag/AgCl) in the presence of the ascorbate, the Au NPs are cleaned from the electrode surface. This fact leads to the conclusion that ascorbate accelerates gold dissolution [62], especially the thin structures as it was found that Au can be dissolved at high potentials in acidic medium. Therefore, further analysis was performed using LSV in the shorter potential range, i.e., from 0 to 0.5 V.

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ACCEPTED MANUSCRIPT Similar behavior was obtained at the AuNPs/ITO/glass electrode with Au NPs of ~220 nm (Fig. 11(b,d)). However, the dependence of the current density on ascorbate concentration is less expressed than that in the case of smaller Au NPs. Thus, the sensitivity of the electrode with the Au NPs of ~220 nm to ascorbate is significantly lower than that of ~90 nm. Generally, the current values at larger Au NPs are ~2.5 lower than at smaller ones. This most

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probably has two reasons: 1) The higher surface-to-volume ratio of smaller Au NPs enhances their electrochemical properties and increases the sensitivity; 2) the lower density of larger Au NPs on ITO glass. Moreover, the dissolution effect was not observed, although also in this case Au oxidation peak was increasing with the amount of ascorbate.

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Calibration curves are shown in Fig. 12 indicate that investigated electrodes with the Au NPs have two linear ranges up to 25 µM and from 50 µM to 150 µM. The sensitivity of ITO electrode with ~90 nm size of Au NPs is ~484 µA/mM (194 µA/(cm2 mM)) up to 25 µM and

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~227 µA/mM from 50 µM to 150 µM. The sensitivity to ascorbate for the electrode with the ~220 nm size of Au NPs was lower, and it was 58 µA/mM (23.2 µA/(cm2 mM)) up to 25 µM and 33 µA/mM with the linear dynamic range up to 1 mM. Limit of detection, calculated using the 3σ method, i.e., 3 times standard error divided into the slope of the calibration curve, was 176 nM and 3.57 µM at ~90 nm and ~220 nm Au NPs electrodes, respectively.

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Such a limit of detection is in the range of concentrations (from 87 nM to 120 µM), obtained with a number of sensors reported for ascorbate detection modified with phenazine dyes and carbon nanotubes [63].

Thus, the results obtained confirm that obtained AuNPs can be employed in electrochemical

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sensing and the smaller are the NPs, the higher is the sensitivity to the analyte. This is most probably related to the effective electroactive surface of the electrode. Moreover, such Au NPs decorated electrodes are stable, and the response signal was the same even after six

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months of the electrode storage. Operational stability depended on the size of the AuNPs, and it was shorter for ~90 nm than those of ~220 nm. In order to compare the response to ascorbate, CVs at the bare ITO electrode were also recorded with different ascorbate additions (Fig. S4). The peak position was much more positive, i.e., at 0.93 V, which is too high for an excellent sensor due to the oxidation of many organic compounds in this potential region. The first linear range was up to 500 µmol/L, and the sensitivity was in between of small and large AuNP modified electrodes, i.e., 54 µA/(cm2 mM). Limit of detection was 1.0 µmol/L.

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this concentration. As seen from the calibration curve (Fig. 13(b)), again 2 linear ranges were observed but the second one has a more significant slope meaning that this method is more sensitive to higher ascorbate concentrations. The sensitivity in this region was ~2.5 times higher than that in the lower concentration region. Nevertheless, it was lower than that by CV.

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Stability of the electrodes using shorter potential window up to 0.5 V was at least half a year, remaining 90 % of an initial response. If the long potential window was used (up to 1.2 V), the stability of the smaller AuNP electrode was less than 100 operational cycles.

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4. Conclusions Summarizing these results, it can be stated that diameters and density of the generated nanoparticles on ITO glass by using nanosecond laser strongly depend on the thickness of the gold film when it is less than 10 nm. The thicker gold film, the larger the diameter, the higher the size dispersion and the lower density of the generated Au NPs on ITO glass are. The exhibited Au NPs on ITO glass method shows the insensitivity to laser processing parameters.

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The surface plasmon resonance was observed only for nanoparticles generated from the thinnest gold films (3 nm and 5 nm). Finally, two ITO electrodes were modified by different size of gold nanoparticles and were characterized electrochemically, using cyclic voltammetry and electrochemical impedance spectroscopy. AuNPs of lower size (~90 nm) had more

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similar electrochemical peculiarities to bare gold. Possible applications were studied for ascorbate determination by cyclic voltammetry and chronoamperometry. The study results

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have shown that the sensitivity of the electrode with the AuNPs of ~90 nm to ascorbate is significantly higher than that of ~220 nm. It can be explained by the higher density and the higher surface-to-volume ratio of smaller nanoparticles on ITO glass. CV was a much more sensitive method for ascorbate analysis than that of chronoamperometry: 194 µA /(cm2 mM) and 8.25 µA/(cm2 mM), respectively. The presented one-step generation of gold nanoparticles on ITO glass by using nanosecond laser method opens a new way for the development of highly sensitive and cost-effective electrochemical sensors. Acknowledgments E. Stankevičius, M. Garliauskas, and R. Trusovas acknowledge to the Lithuanian Research Council for the financial support of the joint Lithuanian-Belarus project in science and 14

ACCEPTED MANUSCRIPT technology “Generation of nanoparticles by laser based methods and formation of structures consisting of nanoparticles by laser” No. S-LB-17-4. N. Tarasenko acknowledges to the Belarusian Foundation for the financial support of Fundamental Researches under Grant No. F17LITG-003. The authors thank, Dr. Paulius Gečys, Dr. Marijus Brikas and Elena Daugnoraitė for technical assistance in all experimental works and valuable suggestions and

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discussions.

References

5. 6. 7. 8.

9.

10.

11.

12. 13. 14.

15.

SC

M AN U

4.

TE D

3.

EP

2.

D. Hernández‐Santos, M. B. González‐García, and A. C. García, Metal‐ Nanoparticles Based Electroanalysis, Electroanalysis 14 (2002) 1225-1235. E. Katz, I. Willner, and J. Wang, Electroanalytical and Bioelectroanalytical Systems Based on Metal and Semiconductor Nanoparticles, Electroanalysis 16 (2004) 19-44. C. M. Welch and R. G. Compton, The use of nanoparticles in electroanalysis: a review, Anal. Bioanal. Chem. 384 (2006) 601-619. F. W. Campbell and R. G. Compton, The use of nanoparticles in electroanalysis: an updated review, Anal. Bioanal. Chem. 396 (2010) 241-259. M. Oyama, Recent Nanoarchitectures in Metal Nanoparticle-modified Electrodes for Electroanalysis, Anal. Sci. 26 (2010) 1-12. M. Oyama and S. Fujita, Competitive Attachment of Gold Nanoparticles on an Indium Tin Oxide Electrode, Anal. Sci. 31 (2015) 597-602. M. T. M. Koper, Structure sensitivity and nanoscale effects in electrocatalysis, Nanoscale 3 (2011) 2054-2073. S. Vaddiraju, I. Tomazos, D. J. Burgess, F. C. Jain, and F. Papadimitrakopoulos, Emerging synergy between nanotechnology and implantable biosensors: A review, Biosens. Bioelectron. 25 (2010) 1553-1565. L. Rassaei, F. Marken, M. Sillanpää, M. Amiri, C. M. Cirtiu, and M. Sillanpää, Nanoparticles in electrochemical sensors for environmental monitoring, TrAC, Trends Anal. Chem. 30 (2011) 1704-1715. Y. Wang, K. R. Ward, E. Laborda, C. Salter, A. Crossley, R. M. J. Jacobs, and R. G. Compton, A Joint Experimental and Computational Search for Authentic Nano‐ electrocatalytic Effects: Electrooxidation of Nitrite and L‐Ascorbate on Gold Nanoparticle‐Modified Glassy Carbon Electrodes, Small 9 (2013) 478-486. X. Yu, L. Wang, and J. Di, Electrochemical Deposition of High Density Gold Nanoparticles on Indium/Tin Oxide Electrode for Fabrication of Biosensors, J. Nanosci. Nanotechnol. 11 (2011) 11084-11088. S. Kinge, M. Crego‐Calama, and D. N. Reinhoudt, Self‐Assembling Nanoparticles at Surfaces and Interfaces, ChemPhysChem 9 (2008) 20-42. Z. Wang and L. Ma, Gold nanoparticle probes, Coord. Chem. Rev. 253 (2009) 16071618. D. Astruc, F. Lu, and J. R. Aranzaes, Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis, Angew. Chem. Int. Ed. 44 (2005) 7852-7872. K. Zhang, J. Wei, H. Zhu, F. Ma, and S. Wang, Electrodeposition of gold nanoparticle arrays on ITO glass as electrode with high electrocatalytic activity, Mater. Res. Bull. 48 (2013) 1338-1341. 15

AC C

1.

ACCEPTED MANUSCRIPT

21. 22. 23. 24.

25.

26.

27. 28. 29. 30.

31.

32. 33.

RI PT

SC

20.

M AN U

19.

TE D

18.

EP

17.

J. Wang, L. Wang, J. Di, and Y. Tu, Electrodeposition of gold nanoparticles on indium/tin oxide electrode for fabrication of a disposable hydrogen peroxide biosensor, Talanta 77 (2009) 1454-1459. J. C. Manifacier, Thin metallic oxides as transparent conductors, Thin Solid Films 90 (1982) 297-308. J. Aldana-González, M. Palomar-Pardavé, S. Corona-Avendaño, M. G. Montes de Oca, M. T. Ramírez-Silva, and M. Romero-Romo, Gold nanoparticles modified-ITO electrode for the selective electrochemical quantification of dopamine in the presence of uric and ascorbic acids, J. Electroanal. Chem. 706 (2013) 69-75. W. A. El-Said, J.-H. Lee, B.-K. Oh, and J.-W. Choi, Electrochemical Sensor to Detect Neurotransmitter Using Gold Nano-Island Coated ITO Electrode, J. Nanosci. Nanotechnol. 11 (2011) 6539-6543. M. Brust, D. Bethell, C. J. Kiely, and D. J. Schiffrin, Self-Assembled Gold Nanoparticle Thin Films with Nonmetallic Optical and Electronic Properties, Langmuir 14 (1998) 5425-5429. T. Okamoto, I. Yamaguchi, and T. Kobayashi, Local plasmon sensor with gold colloid monolayers deposited upon glass substrates, Opt. Lett. 25 (2000) 372-374. A. F. Scarpettini and A. V. Bragas, Coverage and Aggregation of Gold Nanoparticles on Silanized Glasses, Langmuir 26 (2010) 15948-15953. D. Wu, X. Tang, and H. S. Yoon, Deposition of high-density Au nanoparticles on ITO glass by centrifugation, J. Nanopart. Res. 17 (2015) 184. J. Zhang, M. Kambayashi, and M. Oyama, A novel electrode surface fabricated by directly attaching gold nanospheres and nanorods onto indium tin oxide substrate with a seed mediated growth process, Electrochem. Commun. 6 (2004) 683-688. V. Fauzia, N. I. Pratiwi, F. Adela, and D. Djuhana, Controlling the size of gold nanoparticles grown on indium tin oxide substrates prepared by seed mediated growth method, AIP Conf. Proc. 1729 (2016) 020029. M. C. Plante, J. Garrett, S. C. Ghosh, P. Kruse, H. Schriemer, T. Hall, and R. R. LaPierre, The formation of supported monodisperse Au nanoparticles by UV/ozone oxidation process, Appl. Surf. Sci. 253 (2006) 2348-2354. Y. Lin, T. Zhai, and X. Zhang, Nanoscale heat transfer in direct nanopatterning into gold films by a nanosecond laser pulse, Opt. Express 22 (2014) 8396-8404. K. Ratautas, M. Gedvilas, G. Račiukaitis, and A. Grigonis, Nanoparticle formation after nanosecond-laser irradiation of thin gold films, J. Appl. Phys. 112 (2012) 013108. P. Zhaoguang and Z. Xinping, Direct writing of large-area plasmonic photonic crystals using single-shot interference ablation, Nanotechnology 22 (2011) 145303. L. B. Gulina, A. A. Pchelkina, K. G. Nikolaev, D. V. Navolotskaya, S. S. Ermakov, and V. P. Tolstoy, A brief review on immobilization of Gold nanoparticles on inorganic surfaces and Successive Ionic Layer Deposition, Rev.Adv.Mater. Sci. 44 (2016) 46-53. D. V. Timofeeva, Y. V. Tsapko, and S. S. Ermakov, Stripping coulometry determination of lead and mercury at screen-printed electrodes, J. Electroanal. Chem. 660 (2011) 195-199. L. Ding, Y. Gao, and J. Di, A sensitive plasmonic copper(II) sensor based on gold nanoparticles deposited on ITO glass substrate, Biosens. Bioelectron. 83 (2016) 9-14. X. Dai and R. G. Compton, Direct Electrodeposition of Gold Nanoparticles onto Indium Tin Oxide Film Coated Glass: Application to the Detection of Arsenic(III), Anal. Sci. 22 (2006) 567-570.

AC C

16.

16

ACCEPTED MANUSCRIPT

39.

40. 41. 42.

43.

44.

45. 46. 47. 48. 49. 50.

51.

52.

RI PT

SC

38.

M AN U

37.

TE D

36.

EP

35.

L. Wang, W. Mao, D. Ni, J. Di, Y. Wu, and Y. Tu, Direct electrodeposition of gold nanoparticles onto indium/tin oxide film coated glass and its application for electrochemical biosensor, Electrochem. Commun. 10 (2008) 673-676. M. N. Sonuç Karaboğa, Ç. S. Şimşek, and M. K. Sezgintürk, AuNPs modified, disposable, ITO based biosensor: Early diagnosis of heat shock protein 70, Biosens. Bioelectron. 84 (2016) 22-29. X. Liu, C. Fang, J. Yan, H. Li, and Y. Tu, A sensitive electrochemiluminescent biosensor based on AuNP-functionalized ITO for a label-free immunoassay of Cpeptide, Bioelectrochem. 123 (2018) 211-218. X. R. Cheng, B. Y. H. Hau, T. Endo, and K. Kerman, Au nanoparticle-modified DNA sensor based on simultaneous electrochemical impedance spectroscopy and localized surface plasmon resonance, Biosens. Bioelectron. 53 (2014) 513-518. G. Zhu, Q. Song, W. Liu, X. Yan, J. Xiao, and C. Chen, A gold nanoparticle-modified indium tin oxide microelectrode for in-channel amperometric detection in dualchannel microchip electrophoresis, Anal. Methods 9 (2017) 4319-4326. T. Matsui, Y. Kitagawa, M. Okumura, and Y. Shigeta, Accurate Standard Hydrogen Electrode Potential and Applications to the Redox Potentials of Vitamin C and NAD/NADH, J. Phys. Chem. A 119 (2015) 369-376. C. Unger, J. Koch, L. Overmeyer, and B. N. Chichkov, Time-resolved studies of femtosecond-laser induced melt dynamics, Opt. Express 20 (2012) 24864-24872. D. S. Ivanov and L. V. Zhigilei, Combined atomistic-continuum modeling of shortpulse laser melting and disintegration of metal films, Phys. Rev. B 68 (2003) 064114. F. Ruffino, A. Pugliara, E. Carria, L. Romano, C. Bongiorno, G. Fisicaro, A. La Magna, C. Spinella, and M. G. Grimaldi, Towards a laser fluence dependent nanostructuring of thin Au films on Si by nanosecond laser irradiation, Appl. Surf. Sci. 258 (2012) 9128-9137. J. Trice, D. Thomas, C. Favazza, R. Sureshkumar, and R. Kalyanaraman, Pulsed-laserinduced dewetting in nanoscopic metal films: Theory and experiments, Phys. Rev. B 75 (2007) 235439. S. Herminghaus, K. Jacobs, K. Mecke, J. Bischof, A. Fery, M. Ibn-Elhaj, and S. Schlagowski, Spinodal Dewetting in Liquid Crystal and Liquid Metal Films, Science 282 (1998) 916-919. C. Favazza, R. Kalyanaraman, and R. Sureshkumar, Robust nanopatterning by laserinduced dewetting of metal nanofilms, Nanotechnology 17 (2006) 4229. A. Sharma and E. Ruckenstein, Finite-amplitude instability of thin free and wetting films: prediction of lifetimes, Langmuir 2 (1986) 480-494. R. Seemann, S. Herminghaus, and K. Jacobs, Dewetting Patterns and Molecular Forces: A Reconciliation, Phys. Rev. Lett. 86 (2001) 5534-5537. D. Bonn, J. Eggers, J. Indekeu, J. Meunier, and E. Rolley, Wetting and spreading, Rev. Mod. Phys. 81 (2009) 739-805. P. B. Johnson and R. W. Christy, Optical Constants of the Noble Metals, Phys. Rev. B 6 (1972) 4370-4379. E. Stankevičius, M. Garliauskas, M. Gedvilas, N. Tarasenko, and G. Račiukaitis, Structuring of Surfaces with Gold Nanoparticles by Using Bessel-Like Beams, Ann. Phys. 529 (2017) 1700174. P. Gecys, E. Markauskas, M. Gedvilas, G. Raciukaitis, I. Repins, and C. Beall, Ultrashort pulsed laser induced material lift-off processing of CZTSe thin-film solar cells, Sol. Energy 102 (2014) 82-90. J. M. Liu, Simple technique for measurements of pulsed Gaussian-beam spot sizes, Opt. Lett. 7 (1982) 196-198.

AC C

34.

17

ACCEPTED MANUSCRIPT

59.

60.

61.

62. 63.

RI PT

58.

SC

57.

M AN U

56.

TE D

55.

EP

54.

G. Raciukaitis, M. Brikas, P. Gecys, and M. Gedvilas, Accumulation effects in laser ablation of metals with high-repetition-rate lasers, Proc. SPIE 7005 (2008) 70052L. Y. Zhang and J. K. Chen, Melting and resolidification of gold film irradiated by nanoto femtosecond lasers, Appl. Phys. A 88 (2007) 289-297. S. Trasatti and O. A. Petrii, Real surface area measurements in electrochemistry, J. Electroanal. Chem. 327 (1992) 353-376. L. D. Burke and P. F. Nugent, The electrochemistry of gold: I the redox behaviour of the metal in aqueous media, Gold Bull. 30 (1997) 43-53. R. Pauliukaite, M. E. Ghica, O. Fatibello-Filho, and C. M. A. Brett, Electrochemical impedance studies of chitosan-modified electrodes for application in electrochemical sensors and biosensors, Electrochim. Acta 55 (2010) 6239-6247. G. Dryhurst, K. M. Kadish, F. Scheller, and R. Renneberg, in Biological Electrochemistry, G. Dryhurst, K. M. Kadish, F. Scheller, R. Renneberg, Eds., Vol. 1, Ch. 4, Academic Press, Berlin, 1982, pp. 256-278. M. Zidan, T. W. Tee, A. H. Abdullah, Z. Zainal, and G. J. Kheng, Electrochemical Oxidation of Ascorbic Acid Mediated by Bi2O3 Microparticles Modified Glassy Carbon Electrode, Int. J. Electrochem. Sci. 6 (2011) 289-300. S. Chairam, W. Sriraksa, M. Amatatongchai, and E. Somsook, Electrocatalytic Oxidation of Ascorbic Acid Using a Poly(aniline-co-m-ferrocenylaniline) Modified Glassy Carbon Electrode, Sensors (Basel, Switzerland) 11 (2011) 10166-10179. D. X. Silva and A. D. G. Zuniga, Evaluation of the Electro-Catalytic Oxidation of Ascorbic Acid, Using a Platinum Electrode Modified with Poly-3-Methylthiophene, Enciclopédia Biosfera 8 (2012) 1927-2012. S. Cherevko, A. A. Topalov, I. Katsounaros, and K. J. J. Mayrhofer, Electrochemical dissolution of gold in acidic medium, Electrochem. Commun. 28 (2013) 44-46. D. Kul, M. Emilia Ghica, R. Pauliukaite, and C. M. A. Brett, A novel amperometric sensor for ascorbic acid based on poly(Nile blue A) and functionalised multi-walled carbon nanotube modified electrodes, Talanta 111 (2013) 76-84.

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Fig. 1. The principle of Au nanoparticles generation on ITO glass.

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Fig. 2. SEM micrographs of Au coatings affected by single laser pulse when film thickness were 3 nm (a, d), 5 nm (b, e) and 20 nm (c, f). The laser processing parameters in all cases were the same (laser pulse energy – 40 µJ, energy density – 505 mJ/cm2, peak pulse intensity – 14 MW/cm2, pulse duration – 35 ns, beam diameter – 142 µm).

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Fig. 3. The size distribution of Au NPs for different thickness of gold film: a) 3 nm; b) 5 nm; c) 10 nm; d) 15 nm; e) 20 nm; f) 25nm; g) 30 nm; and h) the dependence of the average diameter (black squares) and density (red dots) of generated Au NPs on the different thickness of gold film. Bin size in all histograms is 10 nm. 20

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Fig. 4. Optical density spectra of different gold film thickness (green curve – 3 nm, black curve – 5 nm): a) before laser treatment; b) after single laser pulse treatment when laser energy density was ~ 505 mJ/cm2.

Fig. 5. The size distribution of Au NPs generated in 5 nm thick gold film using single laser pulse with different pulse energy: a) 40 µJ (103 mJ/cm2); b) 80 µJ (205 mJ/cm2); c) 120 µJ (308 mJ/cm2); d) 160 µJ (411 mJ/cm2). Bin size in all histograms is 10 nm.

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Fig. 6. The dependence of average diameter (black squares) and density (red circles) of generated Au NPs on pulse energy (a) and a number of pulses when pulse energy is 80 µJ (b). The thickness of Au thin film is 5 nm.

Fig. 7. The size distribution of Au NPs generated in the 5 nm thick gold film using a different number of laser pulses: a) 1; b) 10; c) 100 and d) 1000 when laser pulse energy was 80 µJ (laser energy density ~ 205 mJ/cm2). Bin size in all histograms is 10 nm.

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Fig. 8. Schematic representation of ITO electrode modification by Au NPs using the laserassisted technique (a); b) an electrochemical cell for cyclic voltammetry experiments. Counter electrode (CE) - Pt wire, reference electrode (RE) - Ag/AgCl electrode, working electrode (WE) - AuNPs/ITO/Glass electrode, electrolyte solution - 0.1 M K2SO4 and 0.01 M H2SO4.

Fig. 9. CVs at bare ITO (pink line), bare Au bulk electrode (green line), AuNP/ITO 5 nm (black line) and 20 nm (blue line) recorded in 0.01 mol/L H2SO4 solution at a potential scan rate of 50 mV/s. b) shows a zoom of current density. Current is normalized per geometric area.

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Fig. 10. Nyquist (a-c) and Cole-Cole (d) EIS spectra in 0.1 mol/L K2SO4 + 0.05 mol/L H2SO4. Complex plane impedance spectra were recorded at bare ITO (a, d), ITO covered with AuNPs from 5 nm (b) and 20 nm (c) gold films at different potentials indicated on plots. Complex plane capacitance spectra were recorded at all films (d) in the double layer region. Grey circles indicate frequencies at maximal impedance/capacitance values.

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Fig. 11. Determination of ascorbate at AuNPs/ITO/glass electrodes with different Au NP size: ~90 nm (generated from 5 nm Au film) (a, c) and ~220 nm (generated from 20 nm Au film) (b, d), using linear sweep voltammetry in 0.1 M K2SO4 solution with successive ascorbate addition (concentrations indicated on plots). Since on metals deprotonized ascorbic acid in ascorbate form responds easier, the analysis was conducted only in K2SO4 solution without addition of sulfuric acid.

Fig. 12. Calibration curves calculated for electrodes AuNPs/ITO/glass with a different size of Au NPs: a) ~90 nm and b) ~220 nm. 25

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Fig. 13. Chronoamperogram of successive addition of ascorbate to electrolyte solution at AuNP(~90 nm)/ITO electrode in 0.1 mol/L K2SO4 + 0.001 mol/L H2SO4 at 0.25 V. Injected concentrations are indicated above the arrows; b) presents a calibration curve obtained from this response.

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ACCEPTED MANUSCRIPT Supplementary materials The identification and the size measurements of the generated Au NPs were performed by processing SEM images with ImageJ program. In Fig. S1 is shown the identification process

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of SEM image by using ImageJ.

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Fig. S1. Identification of generated Au nanoparticles by processing SEM images with „ImageJ“ program: a) laser beam treated area of thin gold film on ITO glass; b) an enlarged view of the area marked with dotted line; c) the pixel conversion into white or black; d) the contours of the identified nanoparticles. The scale bars represent 4 µm. At first, the central part of the laser beam affected area (Fig. S1(a)) was zoomed in (dashed-

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line marked area) in order to get a higher pixel density (Fig. S1(b)). Depending on a pixel brightness value, each pixel of this picture was converted to white or black. In this way, the shadows of the image were eliminated and it was received an image where the white color corresponds to the area occupied by the nanoparticle (Fig. S1(c)). Next step was the identification of the nanoparticles. Since some nanoparticles can be so close to each other that two or more nanoparticles in the picture will have the same overall area, or white areas can be formed only from a few pixels (very small nanoparticles or noise in SEM picture), therefore the boundary conditions were introduced for the improvement the accuracy of the nanoparticles identification. It was registered only such nanoparticles which shape was similar 27

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0.8≤R≤1. The noise was reduced by recording only white areas which correspond to nanoparticles larger than 30 nm in diameter. Also, particles were not measured, which dimensions were outside of the picture. By applying the identification algorithm, it was obtained only the contours of the nanoparticles, which satisfies the defined conditions

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(Fig. S1(d)). The program calculates the area covered by each contour. In order to evaluate the diameter of nanoparticles, each contour area was converted into circles corresponding the

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Fig. S2. CV investigation of the Au NPs with a different number of cycles when Au NPs were obtained from films with a thickness of 5 nm (a) and 20 nm (b).

Fig. S3. Electrooxidation reaction of ascorbic acid in a strongly acidic solution. 28

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Fig. S4. CVs for successive addition of ascorbate to electrolyte solution at ITO electrode (a) and calibration curve obtained from this response (b). Other experimental conditions as in Fig. 10.

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