Light enhanced electrochemistry and electrochemiluminescence of luminol at glassy carbon electrodes

Accepted Manuscript Light enhanced electrochemistry and electrochemiluminescence of luminol at glassy carbon electrodes

Zhiyuan Cao, Bin Su PII: DOI: Reference:

S1388-2481(18)30307-2 https://doi.org/10.1016/j.elecom.2018.11.016 ELECOM 6346

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

3 October 2018 22 November 2018 22 November 2018

Please cite this article as: Zhiyuan Cao, Bin Su , Light enhanced electrochemistry and electrochemiluminescence of luminol at glassy carbon electrodes. Elecom (2018), https://doi.org/10.1016/j.elecom.2018.11.016

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Light enhanced electrochemistry and electrochemiluminescence of luminol at glassy carbon electrodes

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Zhiyuan Cao and Bin Su*

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Institute of Analytical Chemistry, Department of Chemistry, Zhejiang University,

* The corresponding author: Prof. Dr. Bin Su

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Hangzhou 310058, China

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E-mail: [email protected];

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Abstract

We report in this work the enhancement effect of light irradiation on the electrochemistry and electrochemiluminescence (ECL) of luminol at the bare glassy

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carbon electrode (GCE). Light irradiation on the GCE surface induced the formation of

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hot electron-hole pairs. In alkaline solutions, hot holes can oxidize hydroxide anions to hydroxyl radicals that are capable of promoting the oxidation of luminol to generate

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larger current and stronger ECL. Moreover, the stability of current and ECL is also significantly improved upon light irradiation. This study provides a simple method to

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enhance the electrochemistry and ECL of luminol that have promise in improving the sensitivity of luminol-based chemical analysis and bioanalysis.

Keywords: luminol, electrochemiluminescence, glassy carbon electrode, hot hole, light irradiation

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ACCEPTED MANUSCRIPT 1. Introduction Electrochemiluminescence (ECL) is a special form of chemiluminescence (CL) initiated and controlled by electrochemical reactions [1-4]. Due to the excited species are generated at the electrode surface by applying a potential, ECL possesses good temporal and spatial control of luminescence and has a near-zero background compared with

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photoluminescence. As a result, ECL has become a powerful analytical method for molecules detection, biosensors, immunoassay, and clinical diagnostics [5-9]. Luminol is

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one of the most important luminophores that can generate both CL and ECL [10].

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Catalyzed by horseradish peroxidase (HRP), the CL of luminol has been widely used in immunoassay [11]. However, the ECL of luminol is strongly affected by many factors,

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such as pH, applied potential [12, 13] and electrode material [14]. A lot of works have focused on the optimization of electrode material and structure to enhance luminol ECL.

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For example, gold [15-18], silver [19], cobalt nanoparticles [20] and semiconducting materials [21, 22] were modified on electrodes to catalyze luminol ECL and some new electrodes, such as stainless steel [23] and nanoneedle electrodes [24], were also

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developed. However, chemical modification or microfabrication is required in these

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strategies. It remains to be interesting to find a simple method to enhance luminol ECL. Glassy carbon, one of the sp2-hybridized carbon materials, is widely used as

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electrodes in electrochemistry. Lev’s group firstly studied the photocurrent responses of glassy carbon electrode (GCE) and other graphitic materials including boronated glassy

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carbon, carbon black, carbon fiber, and highly oriented pyrolytic graphite. All of carbon materials can generate both anodic and cathodic photocurrents upon UV-visible light irradiation. It has been proposed that hot carriers (electrons and holes) are responsible for the photocurrent generation [25-28]. Herein we report the enhanced current and ECL of luminol by shining a light on the surface of GCE. Light irradiation can induce the formation of hot holes at the electrode surface, which are capable of oxidizing hydroxide anions to hydroxyl radicals to promote the oxidation of luminol, generating larger current and stronger ECL. Moreover, light irradiation also apparently favors the stability of both oxidation current and ECL of luminol. 2

ACCEPTED MANUSCRIPT 2. Experimental The electrochemical and ECL experiments with light shining were carried out on a home-made system based on an upright microscope (Nikon eclipse LV100ND). The schematic of this setup is shown in Fig. 1. An electrochemical workstation (CH660D, Shanghai) was used to supply the electrochemical control. A glassy carbon electrode

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(GCE, 3 mm in diameter), a Pt wire, and an Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively. A mercury lamp (100 W, Nikon C-SHG1)

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was used as light source to light up the working electrode surface. The light beam from

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mercury lamp firstly passed through a long-pass filter (Thorlabs FELH0500), reflected by the bright field cube and then focused on the surface of GCE by a 10× objective. The

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incident light intensity shining on the electrode surface was 460 mW/cm2, as measured by an optical power meter (Thorlabs PM100A). Generated ECL was collected by the

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same objective. A band-pass filter centered at 450 nm (Thorlabs FBH450-10) was used to cut off the excited light. The filtered ECL signal was dispersed by a spectrometer (Andor SR303i-A) and detected by an EMCCD (Andor iXon897). ECL spectra were

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recorded every 200 ms. The ECL intensity at 450 nm was plotted versus time. The

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photoluminescence measurements were performed on a spectrophotometer (Shimadzu

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RF-5301PC).

3. Results and Discussions

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Firstly, we studied the electrochemistry of luminol with and without lighting up the electrode surface. Because protons are involved in luminol electrochemistry, on which the solution pH affects strongly. As shown in Fig. 2a-d, irreversible responses with only anodic current peaks corresponding to luminol oxidation were observed in alkaline conditions. The oxidation potential positively shifted as the solution pH decreased from 13.7 to 10.7. At pH 7.4, the electrochemical process turned to be more reversible with a pair of redox peaks displayed at 0.51 V and 0.42 V. Upon shining the electrode surface with light, an increase of peak current was observed at all pH but more significantly at 3

ACCEPTED MANUSCRIPT alkaline pH. The increased oxidation peak current (∆I) and enhancement ratio (∆I/I0, I0 refers to the peak current without light irradiation) was shown in Fig. 2e. A maximum enhancement was observed at pH 12.5. Fig. 2f compares the photocurrent transient responses at different solution pH. Clearly, a larger enhancement and better stability of photocurrent was obtained at high pH.

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Based on the above results, the mechanism of pH-dependent photocurrent generation was proposed. Glassy carbon is one of the most important amorphous carbon

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materials that consist of nearly 100% sp2-hybridized carbon atoms. When GCE is

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irradiated by light, photons can be absorbed within the space charge layer by π-π* transition and hot electron-hole pairs generate [25]. The hot holes can either recombine

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with the hot electrons or react with luminol, driving hot electrons into the external circuit and generating photocurrent. It has been reported that hydroxyl anions can be oxidized

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to hydroxyl radicals by hot holes, inhibiting the self-recombination of hot electrons and holes [29]. As a result, hot holes can be scavenged much more quickly in highly alkaline solutions, leaving more electrons transferred into external circuit and enhancing the

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photocurrent response (see Fig. 3a). To verify this mechanism, terephthalic acid (TPA)

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was used to capture the hydroxyl radical. TPA can specifically react with hydroxyl radicals to produce 2-hydroxyterephthalic acid (HTPA), yielding a large fluorescent

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enhancement [30]. After being electrolyzed at +0.6 V (vs. Ag/AgCl) under light illumination for 50 minutes, the fluorescence intensity of HTPA increased apparently

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with the incident light intensity (Fig. 3b). Photocurrent transient responses of luminol under different incident light intensity. As shown in Fig. 3c, both photocurrent magnitude and fluorescence of HTPA increased with the incident light intensity. These results confirmed that hydroxyl radicals were responsible for the enhancement of luminol electrochemical oxidation. On the other hand, shining light may increase the temperature at the electrode surface and accelerate the mass transport. To examine the possible temperature effect, we studied the effect of light irradiation on the voltammetric responses of Ru(NH 3)6Cl3 at the glassy carbon electrode. As shown in Figs. 4a and 4b, the voltammetric response 4

ACCEPTED MANUSCRIPT in the dark increased with the solution temperature, however, no obvious change was observed upon light irradiation. Moreover, the effect of light irradiation on the electrochemistry of luminol at the indium tin oxide (ITO) electrodes was also investigated. As displayed in Figs. 4c and 4d, light irradiation on ITO electrodes had no enhancement effect on either voltammetric or amperometric responses. Note that ITO does not absorb light in the current wavelength range, therefore no hot carriers were

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generated upon light irradiation. These results can prove that the temperature effect does

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not contribute to the current enhancement.

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The enhancement effect of light illumination on luminol ECL was investigated in 0.1 M NaOH containing 5 mM luminol (pH = 12.5). As shown in Fig. 5a, the peak current

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decreased by 42% after 25 cyclic voltammetric cycles under the dark condition. However, only 8% of current decline was observed under light irradiation. Moreover, ECL intensity

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of luminol also showed an enhanced stability under light illumination (see Fig. 5b), maintaining a stronger magnitude over 25 voltammetric cycles. In contrast, ECL signal under the dark condition nearly disappeared after 10 voltammetric cycles (Fig. 5b, inset).

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Fig. 5c compared the ECL intensities at different concentrations of luminol with and

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without light irradiation. Upon lighting up the electrode, ECL integral intensity over 10 CV cycles increased more sharply with increasing luminol concentration from 0.1 mM to 2 mM. Finally, the effect of changing incident light intensity on the voltammetric peak

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current and ECL of luminol was studied. Due to hydroxyl radicals generated more quickly under brighter incident light, both of them increased with increasing the light

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power. ECL enhancement factor, defined as the ratio of ECL integral intensity over 10 CV cycles with illumination to ECL integral intensity over 10 CV cycles without illumination, displayed a maximum of 4.2 at a light intensity of 460 mW/cm2. The mechanism of improved current and ECL stability remains unclear and will be studied in future.

4. Conclusions In summary, our work has demonstrated the pH-dependent enhancement of luminol 5

ACCEPTED MANUSCRIPT ECL at bare glassy carbon electrodes by photo-induced hot carriers. In highly alkaline solutions, hydroxyl anions can be oxidized by hot holes to produce hydroxyl radicals, which results in a much larger anodic current enhancement. This method can also be applied to enhance the luminol ECL. Both ECL intensity and stability are greatly enhanced upon light irradiation. This work represents a simple method to enhance the luminol ECL in the absence of H2O2 that has potential applications in ECL-based

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analysis and bioanalysis.

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Acknowledgements

The financial support by the Nature Science Foundation of China (21335001 and

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21575126) and the Nature Science Foundation of Zhejiang Province (LZ18B050001) is

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gratefully acknowledged.

Conflicts of interest

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None.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Home-made experimental system for electrochemical and ECL measurement.

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Fig. 2. (a-d) Cyclic voltammograms (CVs) of 5 mM luminol at a bare GCE with (blue) and without (red) light irradiation at different solution pH (from a to d: pH = 7.4, 10.7, 12.5, and 13.7). The scan rate was 50 mV/s. (e) Peak current enhancement (∆I) and enhancement ratio (∆I/I0) of three parallel experiments versus the solution pH. (f) Photocurrent transient responses at the oxidation potential of 5 mM luminol in different pH systems, from bottom to top, the potentials were 0.51 V, 0.55 V, 0.26 V, and 0.18 V (vs. Ag/AgCl).

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Fig. 3. (a) Schematic illustration of photoexcitation of bare GCE in alkaline solutions. (b) Fluorescence spectra of 5 mM TPA in 0.1 M NaOH after being electrolyzed at +0.6 V (vs. Ag/AgCl) under different incident light intensity for 50 minutes. The excitation wavelength was 312 nm. (c) Fluorescence intensity at 430 nm of TPA and photocurrent enhancement of 5 mM luminol in 0.1 M NaOH at +0.3 V (vs. Ag/AgCl) versus the incident light intensity.

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Fig. 4. (a) CVs of 0.5 mM Ru(NH3)6Cl3 at a bare GCE in 0.1 M KCl at different temperatures. (b) CVs of 0.5 mM Ru(NH3)6Cl3 at a bare GCE in 0.1 M KCl at 20 oC with (blue) and without (red) light irradiation. (c) CVs of 5 mM luminol at a bare ITO in 0.1 M NaOH with (blue) and without (red) light irradiation. The potential scan rate for all CVs was 50 mV/s. (d) Chronoamperometric curve of 5 mM luminol at a bare ITO in 0.1 M NaOH with periodic light on and off. The applied potential was +0.7 V (vs. Ag/AgCl).

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Fig. 5. CV (a) and ECL (b) of 5 mM luminol in 0.1 M NaOH with (blue) and without (red) light illumination. Inset of b: ECL intensity of the 8th to 11st voltammetric cycles with (blue) and without (red) light illumination. (c) ECL integral intensities over 10 CV cycles at different concentrations of luminol with (blue) and without (red) light irradiation. (d) Peak current enhancement (red) and ECL integral intensity (blue) as a function of the incident light intensity. The potential scan rate for all CVs was 50 mV/s.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 

Photocurrent was observed with luminol at bare glassy carbon electrode (GCE).

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Illuminated GCE showed sensitizing effect on hydroxyl radical production.

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Electrochemiluminescence (ECL) of luminol was greatly intensified by lighting up the GCE.

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Light irradiation favored the stability of oxidation current and ECL of luminol.

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