2-Dimensional electrochemiluminescence on porous silicon platform

and electron transfer ability. The detection and discrimination of explosives1 is critical in global security and great efforts have been devoted to d...
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2-Dimensional electrochemiluminescence on porous silicon platform for explosives detection and discrimination Yaoxuan Cui, Yao Jin, Xisheng Chen, and Jianmin Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00113 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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2-Dimensional electrochemiluminescence on porous silicon platform for explosives detection and discrimination Yaoxuan Cui, Yao Jin, Xisheng Chen and Jianmin Wu* Department of Chemistry, Zhejiang University, Hangzhou, China. KEYWORDS: Porous silicon; Gold nanoparticle; 2-Dimensional electrochemiluminescence; Explosive; Discrimination

utilized to monitor electrochemical reaction in the pore channels of pSi. The large solid-liquid interface can also provide a convenient way to construct a sensor array with multiple reaction sites. Up to now, quantum confined effect and surface oxidation state mechanism have been proposed to explain pSi-ECL. Since Allen Gee first discovered the ECL phenomenon of pSi in 19608, there is still some controversies on the ECL mechanism. Our previous work has indicated that surface oxidative state plays a vital role in pSi ECL process7. The dynamic process of ECL can be remarkably altered by electron acceptors or donators due to the charge transfer at the pSi-absorbates interface. The electron donors or acceptors can significantly affect the ECL intensity by changing the possibility of radiative electron-hole recombination. Using this approach, compounds with electron donating or withdrawing ability can be effectively discriminated by pSi-ECL. However, quenching of pSi-ECL can be not only caused by electron transfer but also caused by surface oxidation. Therefore, merely measuring the ECL intensity cannot distinguish the two quenching mechanism. This shortcoming may also exist in most of quenching-based ECL approach. Herein, a 2-D ECL approach was established to distinguish and detect three classes of explosives. Among them, nitro compounds can quench ECL intensity without affecting the initial ECL peak position due to the electron-withdrawing effect of nitro group; peroxides explosive without N can quench ECL intensity owing to the pre-oxidation of Si-H bond, which is a main electron source of pSi-based ECL. In the meantime, pre-oxidation of pSi can also introduce a surface defect energy level, causing the blue shift of initial ECL peak; In contrast, when the pre-oxidation was caused by peroxides with amine group, the ECL spectra display initial peak shift but the decrease in peak intensity is not so significant since the offset effect resulted from surface oxidation and electron-donating ability of amine group. By simultaneously monitoring the response of peak position and intensity to different chemical stimulation, the 2D ECL strategy can detect and discriminate oxidants, electron donators and electron acceptors. Furthermore,

ABSTRACT: This work established a rapid and sensitive

explosive detection and recognition technique. We report a two-dimensional electrochemiluminescence (2-D ECL) method based on porous silicon (pSi) by monitoring the dynamic change in peak position and peak intensity of pSi-ECL. Gold nanoparticles (AuNPs) were deposited on the pSi surface to promote the electrochemical reaction and electron transfer efficiency at the pSi-electrolyte interface. The 2-D ECL can effectively detect and discriminate different classes of explosives including nitro compounds, peroxides with nitrogen atoms and peroxides without nitrogen atoms due to their different oxidation and electron transfer ability.

The detection and discrimination of explosives1 is critical in global security and great efforts have been devoted to develop rapid, sensitive, selective sensing methods for detecting explosives based on optical2 and electrical technology3. Up to now, there are three major classes of explosives including nitro compounds, peroxides with nitrogen atom and peroxides without nitrogen atom. However, most optical sensing strategies only measure intensity signal caused by quenching or enhancing mechanism. Accordingly, those different classes of explosives cannot be discriminated if only the optical intensity was measured. Among various optical sensors, ECL is a powerful and sensitive technique that has been extensively used in bioanalysis and hazardous chemicals detection4. In contrast to fluorescence, ECL can be precisely controlled by applied potential without need of external light source. Thus, ECL sensors have simplified instrumental setup, low background noise and high reproducibility5. However, conventional ECL methods usually need co-reactants during electrochemical reations6. Our previous study has demonstrated that pSi may become a label-free ECL sensing material, because of its unique surface dependent behavior and co-reactant free property7. In addition, most of pSi-ECL take place in pore channels of pSi, therefore, pore open or pore blockage strategies can be potentially

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The newly etched “dark” pSi can emit photoluminescent (PL) after 5 cyclic voltammetric scanning. The peak wavelength of PL spectra also undergoes a blue shift during the continuous voltammetric scanning. As indicated in Figure 1A, the peak position of ECL spectra shift from

ECL and PL spectra9-11 were employed to investigate the different ECL quenching mechanism. To achieve better sensitivity, AuNPs were deposited on pSi surface to promote the electrochemical reaction and electron transfer efficiency at pSi-electrolyte interface. The utilization of AuNPs was inspired by the enhanced photoluminescence12, SERS13 and MALDI14 on the AuNPs-Si binary hybrids due to the catalytic ability15 and LSPR effect14 of AuNPs, depending on its size13 and shape16. AuNPs were also combined with SiO2 nanoparticles17 to obtain better ECL based detection sensitivity. Detailed methods and a schematic workflow for the preparation and use of pSi are provided in SI (Scheme S1). PSi contains a large amount of Si nanocrystals distributing on its surface. Before electrochemical reaction, the

Scheme 1. Energy level of pSi during ECL process

Figure 1. (A) ECL spectra during 1-14 voltage scanning cycles; (B) PL spectrum of pSi after 5 (black) and 14 (red) voltage cycles; (C) ECL spectra of AuNPs-pSi during 1-8 voltage scanning cycles; (D) Comparison of maximum ECL intensity after reacting with H2O2 for different time on pSi and AuNPs-pSi chips; (E) ECL image of pSi after reacted with H2O2 of 0, 1, 10 and 100mM for 5 min and (F) 60min, (G) ECL image of AuNPs-pSi after reacted with H2O2 of 0, 1, 10 and 100mM H2O2 for 5 min.

conduction band (CB) and valence band (VB) of Si nanocrystal have an indirect bandgap, which displays a low efficiency of radiative recombination between electrons and holes and low luminescent quantum yield. The oxidation of Si-Hx bonds introduces surface state, leading to a transition from the indirect bandgap between Si cores to direct bandgap between surface states. The high efficiency of electron-hole recombination at surface state results in the increase of ECL intensity during the initial 5 scanning cycles (Figure 1A). Thereafter, ECL intensity gradually decreases, which can be ascribed to the consuming of Si-Hx bonds and the increasing of the thickness of SiO2 shell layer, which may block the electron transfer between pSi and electrolyte. The blue shift of ECL peak position during ECL process should be caused by the reduction in the core size of Si nanocrystal, leading to the expanding of bandgap between surface state energy levels or the CB~VB energy levels, as indicated in Scheme 1.

745 nm to 697 nm during the 5th to 14th scanning cycles, while the PL spectra shift from 681 nm to 653 nm (Figure 1B). Compared to PL, the peak wavelength of ECL is much longer, indicating that ECL and PL have different electron transition channels. Usually, surface state and quantum confined electronic energy level of Si are the two major channels contributing to the luminescent emission of Si nanocrystals18. For ECL emission, electron transition between surface state is a major factor9, since ECL takes place at the interface between pSi and electrolytes. In contrast, the PL emission mainly relate to the quantum confined energy level of Si core. As illustrated in Scheme 1, surface state energy levels are always between the CB~VB energy level of Si cores. The blue shift of PL spectrum indicates that Si cores gradually shrink during oxidation, causing the widen of bandgap due to quantum confined effect during surface oxidation of pSi. Although mild oxidation of pSi can introduce surface state to increase the quantum yield of radiative recombination, relevant studies have found that over-oxidization may introduce SiO2 structure to Si nanocrystals, leading to the increase of non-radiative recombination and quenching of PL19. A similar quenching phenomenon was also observed in pSi-ECL if it was oxidized by peroxide. The decreased ECL intensity is proportional to the concentration of H2O2 after reacted with pSi for 60 min (Figure S1d). DR-FTIR spectra of pSi reacted with 10mM and 100mM H2O2 (SI Figure S1e,) shows two distinct peaks at 2110 cm-1 and 1070 cm-1 corresponding to the SiHx bonds and Si-O-Si bonds, respectively20. However, unlike the electrochemical-oxidized samples, the peak corresponding to O-Si-Hx bonds did not appear in the range of 2200-2300 cm-1, indicating that the SiHx bonds were totally oxidized to Si-O-Si bonds by peroxide. Thus, it can be inferred that the oxidation-induced ECL quenching may result from the introduction of Si-O-Si surface defect and reduction in the amount of Si-Hx bonds in the electrochemical reaction. Although, the difference in the mechanism and kinetics between aqueous and dry oxidation of pSi have been investigated before21, this work pro-

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posed different mechanism to explain different ECL behavior caused by mild and strong oxidation of pSi. Initial electrochemical oxidation leads to the enhancement of ECL intensity, while H2O2 oxidation leads to quenching of ECL intensity. The origin of the phenomenon is ascribed to the different oxidative products of Si-Hx bonds. In electrochemical oxidation, both surface state and defect were simultaneously introduced because Si-Hx bonds were oxidized to O-Si-Hx and Si-O-Si bonds, as indicated in DR-FTIR spectra. When the kinetic process of radiative recombination at surface state is predominant, the ECL intensity will be enhanced. As to the H2O2 oxidation, Si-Hx bonds were totally oxidized to Si-O-Si bonds, which acted as surface defect and quenched the ECL intensity (Scheme S2). In addition, both surface oxidative state and defect can lead to the initial blue shift in peak position due to the reduction of Si core size and the widened surface state energy level. To improve the ECL performance of pSi, AuNPs were electrochemically deposited on the pSi, following the method reported in our previous work14. The condition for AuNPs deposition can severely affect the ECL behavior of pSi. The maximal ECL intensity can be achieved when the deposition time and HAuCl4 concentration was controlled at 30s and 0.03%, respectively (Figure S2, S3). SEM images of AuNPs-pSi prove that AuNPs have been uniformly deposited on the surface of pSi. When the deposition time is 30s, 60s and 90s, the surface coverage of pSi by AuNPs is around 26%, 58% and 72% with standard deviation around 3%, respectively, indicating that more surface area of pSi was covered by AuNPs with increasing deposition time (Figure S4). Fourier transformed reflectointerference spectrum (FT-RIS) spectra show that the effective optical thickness(EOT) of pSi samples shifts from 12986 to 11854 nm after the deposition of AuNPs for 120s (Figure S5). The blue shift of EOT might be caused by the surface oxidation of pSi and most of AuNPs were coated on the inner surface wall of pSi. Due to nAu and nSiO2 are less than nSi 22, the average refractive index decease accordingly. Different surface chemistry of pSi before and after AuNPs deposition can be further confirmed by peaks of Au 4f, Au 4d and O 1s in XPS (Figure S6). Compared to pSi, ECL intensity of AuNPs-pSi reached the highest in the first cycle of voltammetric scanning, and its intensity increased more than 5 folds (Figure 1C). The promoted ECL performance may result from catalysis effect15, hydrophilic property and LSPR effect of AuNPs14. Contact angle measurements show that the contact angle of water droplet on pSi and AuNPspSi is ~ 56° and 22°, respectively, indicating that the AuNPs-pSi sample is more hydrophilic. Since most of ECL take place at the pore channel of pSi, the hydrophilic surface is in favor of increasing the pore accessibility to electrolyte (Figure S7). The CV curves of pSi and AuNPs-pSi proved the catalytic effect of AuNPs during

voltammetric scanning (Figure S8). When cyclic voltage scanning from -0.1V to 1V was applied, the oxidation peak of pSi appears at 0.33V only after the third cycle. As for the AuNPs-pSi sample, an obvious oxidation peak can be observed in the first voltage cycle at 0.23V, which is around 0.1 V negative shift compared to pSi. The earlier appearance of oxidation peak indicates that AuNPs can lower the over potential of the electrochemistry reaction, thus act as a powerful catalyst in electrochemistry reactions. In addition, the AuNPs can also catalyze the oxidation reaction between pSi and H2O2. For the bare pSi sample, it needs 60 min to show intensity difference in the ECL image among pSi samples pre-oxidized with 0, 1, 10 and 100mM H2O2. In contrast, it only took 5min for the AuNPs-pSi to achieve the similar result (Figure 1E-G). Thus, AuNPs can not only enhance the ECL intensity, but also catalyze the reaction between pSi and oxidants. The present work found that 2-D ECL acquired on AuNPs-pSi can effectively detect and discriminate three major types of explosives: nitro compounds, peroxides

Scheme 2. Discrimination principle of three explosives

with N and peroxides without N, because the three types of explosive may display different mechanism to affect the ECL of pSi, as illustrated in Scheme 2. TNT, TATP, HMTD are typical explosives of three major classes. After reacting AuNPs-pSi with these compounds with concentration of 10, 30, 50 μg/mL for 30mins, ECL image (Figure 2A-C) and spectra of AuNPs-pSi (Figure 2D-F) was simultaneously acquired by CCD camera and fiber spectrometer (Ocean Optics, QE Pro). Compared to the ECL intensity of pSi pretreated with solvent (methanol aqueous), TNT can effectively quench ECL of AuNPspSi because of electron withdrawing effect of nitrogroups during ECL process. For other typical nitro-compounds explosives are DNT and NT, similar phenomenon can be observed in ECL image and spectra of AuNPs-pSi (Figure S9b, c). In addition, the quenching efficiency increase as the number of nitro-groups increase for the same concentration of TNT, DNT and NT (Figure S9g). TATP was chosen as typical peroxide explosives without nitrogen atoms. ECL intensity of AuNPs-pSi can be quenched by pre-oxidation before ECL process, owing to the decrease in the amount of Si-Hx bonds on pSi surface and

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the introduction of surface defect (Figure 2B). HMTD is also a peroxide explosive with two nitrogen atoms in each molecule. However, when AuNPs-pSi react with different concentration of HMTD, the quenching of ECL intensity is not significant (Figure 2C), even though the pre-oxidation of pSi should take place since the molecule also contains peroxy groups. To quantify the quenching effect, the captured ECL images were processed with the ImageJ2x software. The gray values of the quenched area and background can be obtained, the quenching factor (Q) is calculated from the following equation: Q = (G0-G)/G0 (eq. 1) in which G denotes the gray value of the pSi reacted with explosives and G0 the gray value of the reacted with solvent. The Q value displays an almost linear relationship with TNT and TATP concentration (Figure 2H) and H2O2 concentration (Figure S9c). On the basis of the gray-value calculation, the detection limit (S/N > 3) for TATP and TNT molecules is around 10 and 1 μmol/L, respectively. Compared to our previous work7, AuNPs-pSi display better sensitivity in a shorter reaction time.

Figure 2. (A-C) ECL image of AuNPs-pSi after reacted with TNT(A), TATP(B), HMTD(C) of 0, 10, 30 and 50 μg/mL for 5 min; (D-F) ECL peak position changes between 1 and 2 voltage scanning cycle of TNT, TATP, HMTD of 50 μg/mL; (G) 2-dimensional plot composed by relative gray value and relative R value of AuNPs-pSi reacted with TNT, TATP, HMTD of 0, 10, 30 and 50 μg/mL; (H) Relative gray value of three ECL images; (I) Relative R value of AuNPs-pSi after reacted with TNT, TATP, HMTD of 0, 10, 30 and 50 μg/mL.

Pre-oxidation before ECL can result in the pSi surface oxidation and consequently cause the reduction of Si core and blue shift of initial ECL peak. To evaluate the preoxidation level of pSi, the initial peak shift rate (R) is calculated by the following equation: R= (λ1th-λ2th)/ (λ1th, control -λ2th, control) (eq. 2) where R value represents the ratio between the peak shift in the first cyclic scanning and the overall peak shift. λ1th

and λ2th refer to the maximum wavelength of the ECL peak measured in first and second cyclic voltammetric scanning, respectively, λ1th, control and λ2th, control refer to the maximum wavelength of the first and second ECL peak measured on the pSi treated with blank sample, respectively. In the 2-D ECL, the R value calculated from the data of initial peak shift can reveal whether a pre-oxidation process took place on the pSi. For TNT (Figure 2I), DNT and NT sample (Figure S9i), the R value of Au-pSi is nearly 1.00, which is almost identical to the blank AuNPs-pSi. These results indicated that nitro-aromatic compounds can only accept the electro-excited electron without a surface oxidative process. Pre-oxidation of AuNPs-pSi before ECL measurements could introduce surface state level and reduce the Si core size, leading to the blue shift of initial ECL peak and decrease in the R value. According to the eq. 2, a smaller R value is an indicative of higher oxidative ability of oxidants. The relative R Value of AuNPs-pSi reacted with TATP and H2O2 of 225 μmol/L is 0.439 and 0.809, respectively. This phenomenon can be explained by the energy level diagram of oxidants. TATP has a higher redox potential than that of H2O2, while the redox potential of Si-H has a wide distribution in energy level due to the heterogeneity of Si nanocrystal. As a result, TATP can oxidize more Si-H bonds on the surface of AuNPs-pSi. Thus, the R value is a characteristic parameter to discriminate oxidants with different oxidative ability (Figure S9c). The R value of AuNPs-pSi pretreated with HMTD of 50μg/mL is 0.444(Figure 2I), indicating that surface oxidation also occurred before the ECL process. Nevertheless, the electron-donating effect of nitrogen atoms in the HMTD can enhance the ECL efficiency and thereby offset the oxidation-induced quenching effect. By simultaneously monitoring the peak position and peak intensity of ECL, a 2D ECL approach can be thereby established. In the present work, the relative gray value and R value was used as coordinate to compose a 2-D plot, in which each type of explosive with different concentration appear in different pattern (Figure 2G). Accordingly, those explosives with different chemical structures can be discriminated with the 2-D ECL strategy. In summary, a 2-D ECL approach has been proposed by simultaneously measuring the peak intensity and position of ECL on the AuNPs-pSi, in which AuNPs can accelerate both chemical reaction and electrochemical reaction. The surface oxidation during ECL process will lead to introduction of surface state energy level and reduction in the Si core size, thereby causing the blue shift of ECL peak during cyclic voltammetric scanning. Pre-oxidation of pSi with oxidants can introduce the surface defects and accordingly not only quenching the ECL intensity but also decrease the rate of initial peak shift comparing with the blank pSi. In contrast, explosives containing nitro group can just quench the ECL of pSi through electron

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transfer process but without a pre-oxidative effect, whereas compounds with electron donating ability （e.g. amine group）can enhance the ECL intensity. However, if this compound also contains peroxy group, the quenching and enhancing effect might be counteracted. Compared with conventional ECL, the 2-D ECL can provide more insight to the inherent characteristic of chemical compounds. As a proof-of concept, this work successfully utilized the 2-D ECL to discriminate different types of explosives. The application of this technology can potentially be extended to chemical sensors for the detection of various types of reactive oxidation species, antioxidants or the activity of oxidative enzymes. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed materials and methods; supplementary data referenced throughout text (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Fax: +86 571 88273572; Tel: +86 571 88273496. Present Addresses

Department of Chemistry, Zhejiang University, Hangzhou, China. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (No. 21575127), and the Natural Science Foundation of Zhejiang province (No. Z15B050001)

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