Dual Enhanced Electrochemiluminescence of Aminated Au@SiO2

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Dual Enhanced Electrochemiluminescence of Aminated Au@SiO2/ CdS Quantum Dot Superstructures: Electromagnetic Field Enhancement and Chemical Enhancement Xueyuan Li, Yang Xu, Yuxia Chen, Chen Wang, Jingjing Jiang, Jiangtao Dong, Hua Yan, and Xuezhong Du*

Downloaded via UNIV OF GOTHENBURG on January 23, 2019 at 14:18:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Key Laboratory of Mesoscopic Chemistry (Ministry of Education), State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life Sciences, and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China S Supporting Information *

ABSTRACT: This paper reports dual enhanced electrochemiluminescence (ECL) of CdS quantum dot (QD)-decorated aminated Au@SiO2 core/shell (Au@SiO2-NH2/CdS) superstructures. A maximum ECL emission of the Au@SiO2-NH2/CdS superstructures (Au core, ca. 55 nm) with a silica shell of 38 nm was 35-fold stronger than that of the counterparts (containing neither Au cores nor amino groups) with H2O2 as a coreactant. The fold of ECL enhancement is the largest, and the optical path of maximum ECL enhancement is the longest reported so far. The larger the Au cores in the superstructures, the stronger the ECL emission of CdS QDs was. Two types of ECL enhancement mechanisms were clearly proposed for the dual enhanced ECL of the Au@SiO2-NH2/CdS superstructures. One was the electromagnetic field enhancement induced by localized surface plasmon resonance of Au cores, and the other was the chemical enhancement from amino groups modified on the silica surface involved in the ECL process in the assistance of H2O2. It is the first time to put forward the new concept of chemical enhanced ECL that was directly related to the participation of other chemicals, which caused a decrease in the difference in the redox potential between emitters and coreactants for the increase of their redox currents. The constructed ECL platform was demonstrated to have promising applications in highly sensitive detection of glutathione (GSH), and the response mechanism of GSH was also explored. KEYWORDS: CdS quantum dot, Au@SiO2 nanoparticle, electrochemiluminescence enhancement, electromagnetic field enhancement, chemical enhancement, electrochemiluminescence detection

■

(FRET) and electromagnetic field enhancement.6 A maximum surface enhanced fluorescence (SEF) is achieved at a certain distance from the nanostructured surface. Tian and co-workers used Au nanoparticles (AuNPs, ca. 55 nm) coated with ultrathin and nonporous silica shells (2−4 nm) to develop the shell-isolated NP enhanced Raman spectroscopy to prevent direct contact between analytes and nanostructured metallic surfaces.7 Acroca and Guerrero used the Au@SiO2 core/shell NPs (Au core, 45 nm; silica shell, 20 nm) to obtain the SEF spectra of dye-derived amphiphiles in the monolayers.8 The silica shell as a spacer layer was used to adjust the distance between the analytes and metallic surfaces. We reported bifunctional superstructures of CdS nanocrystal (quantum dot, QD)-decorated Ag@SiO2 NPs embedded with the apparent Raman probe, p-aminothiophenol, for simultaneous SERS and

INTRODUCTION The localized surface plasmon resonance (LSPR) effect of noble metal nanostructures can cause strongly enhanced electromagnetic fields. The spectral signals of analytes located within the enhanced electromagnetic fields can be significantly affected. Surface enhanced Raman scattering (SERS) spectroscopy is dominantly contributed from the enhancement of electromagnetic fields, associated with the excitation of collective electron oscillations of noble metal nanostructures, as well as chemical enhancement,1−4 which can lead to the order-of-magnitude increase of molecular Raman scattering cross-sections. However, SERS signals are exponentially attenuated with the increase of distance of analytes away from nanostructured metallic surfaces. If the analytes contain fluorophores, a continuous transition from fluorescence quenching to fluorescence enhancement takes place,5 dependent on the distance of fluorophores from nanostructured metallic surfaces because of the competition between nonradiative damping due to the Förster resonance energy transfer © XXXX American Chemical Society

Received: August 28, 2018 Accepted: January 3, 2019 Published: January 3, 2019 A

DOI: 10.1021/acsami.8b14886 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Illustration of Preparation of CdS QD-Decorated Aminated Au@SiO2 Superstructures, Construction of Dual Enhanced ECL Platform, and ECL Enhancement Mechanisms

55 nm) were constructed for dual enhanced ECL platform (Scheme 1). The modified amino groups on the surfaces of silica shells not only facilitated adsorption of CdS QDs onto the surfaces of Au@SiO2 NPs but also promoted further ECL enhancement. The maximum ECL emission of the Au@SiO2NH2/CdS superstructures (Au core, ca. 55 nm) with a silica shell of 38 nm was 35-fold stronger than that of the counterparts (containing neither Au cores nor amino groups). To the best of our knowledge, the fold of ECL enhancement is the largest and the optical path of maximum ECL enhancement is the longest reported so far. Two types of ECL enhancement mechanisms were clearly proposed for the dual enhanced ECL: one was the electromagnetic field enhancement induced by the LSPR of AuNPs, and the other was the chemical enhancement from modified amino groups, which were first oxidized by H2O2 and then electrochemically reduced. It is the first time to put forward the new concept of chemical enhanced ECL that was directly related to the participation of other chemicals, which caused a decrease in the difference in the redox potential between emitters and coreactants for the increase of their redox currents. The constructed ECL platform from the Au@SiO2-NH2/CdS superstructures was demonstrated to have promising applications in highly sensitive detection of glutathione (GSH), and the response mechanism of GSH was also explored.

SEF immunoassays, and a maximum SEF intensity of the decorated CdS QDs was available with a silica shell of 9 nm.9 Electrochemiluminescence (ECL) is the process whereby species generated at electrode surfaces undergo electron transfer reactions to form excited states that emit light without the need of an external light source.10−13 Similarly, if ECL emitters are located in the vicinity of nanostructured noble metals, surface enhanced ECL (SEECL) emission can be achieved.14−16 The ECL emission of the emitters can excite LSPR of proximal nanostructured noble metals to generate electromagnetic fields, which can ultimately enhance ECL emission of the emitters.14,15 Xu and co-workers fabricated a modified electrode first with the film of CdS QDs (5 nm), then with double-stranded DNA (12 nm long) as a linker, and finally with AuNPs (5 nm); thus, the cathodic ECL emission of CdS QDs in the modified electrode with S2O82− as a coreactant was 5-fold stronger than that in the absence of AuNPs.17 Wu and co-workers fabricated the same ECL platform as described above, and the ECL emission of CdS QDs was 9-fold stronger than that in the absence of AuNPs when the double-stranded DNA linkers were ca. 14 nm long.18 However, the AuNPs smaller than 5 nm are known to possess catalytic activity. Guo and Lin and co-workers fabricated modified electrodes with Au@SiO2 NPs (Au core, ca. 25 nm) with the silica shells of different thicknesses, and thus the best anodic ECL enhancement (ca. 8-fold) of Ru(bipy)32+ with tripropylamine (TPrA) as a coreactant was obtained with a silica shell of 6 nm.19 Furthermore, Guo and co-workers fabricated a modified electrode with a multilayer of Ru(bipy)32+-doped silica NPs (55 nm) and AuNPs (45 nm) nanoarchitectures modified with two different kinds of aptamers specific to antigens; thus, 30-fold ECL enhancement could be obtained due to the multilayer SEECL effect.20 Various kinds of semiconductor QDs have been chosen as promising ECL emitters in sensing applications.17,18,21−24 Among them, CdS QDs are one of the most studied electroluminescent materials owing to simple surface functionalization and tunable ECL properties. The size and shape of AuNPs are related to the magnitude of electromagnetic field generated by the LSPR of AuNPs. The SEECL emission of CdS QDs in the presence of AuNPs should be dependent on the distance between them, which could be well tuned by the thickness of silica shells coated around Au cores as well as ease of silica modification. Herein, CdS QD-decorated aminated Au@SiO2 (Au@SiO2-NH2/CdS) superstructures (Au core, ca.

■

EXPERIMENTAL SECTION

Chemical Reagents and Materials. Chloroauric acid (HAuCl4· 4H2O) was purchased from Shanghai Chemical Reagent Co., Ltd. (China). Sodium citrate (99%), ammonia (NH3·H2O), anhydrous ethanol, H2O2 (30%), concentrated H2SO4 (98%), isopropyl alcohol, Na2S, CdCl2, NaOH, NaCl, KCl, and CuCl2 were obtained from Nanjing Chemical Reagent Co., Ltd. (China). Selenium powder and Na2SO3 were purchased from Energy Chemical Industry Co., Ltd. (China), and thioglycolic acid (TGA) from Shanghai Lingfeng Chemical Reagents Co., Ltd. (China). Reduced GSH (98%) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (China), and oxidized GSH (GSSG, ≥98%) from Shanghai Yuanye Bio-Technology Co., Ltd. (China). Alanine, glucose, fructose, cysteine, dopamine, ascorbic acid, uric acid, and AgNO3 were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Tetraethylorthosilicate (TEOS), 3-aminopropyltriethylsilane (APTES), and 3-mercaptopropyltrimethoxysilane (MPTMS) were purchased from Sigma-Aldrich, and N-trimethoxysilylpropyl-N,N,Ntrimethylammonium chloride (50% in methanol) from J&K China Chemical Ltd. (Shanghai, China). The aqueous solutions used were prepared with double-distilled water with a resistivity of 18.2 MΩ cm. B


Research Article

ACS Applied Materials & Interfaces The phosphate buffered saline including Na2HPO4 and NaH2PO4 containing 150 mM NaCl [phosphate-buffered saline (PBS), 10 mM, pH 7.4] was prepared in double-distilled water. Instruments. TEM and SEM images were acquired on a JEOL JEM-2100 transmission electron microscope and a Hitachi S-4800 scanning electron microscope, respectively. X-ray diffraction (XRD) patterns were measured on a D8 ADVANCE X-ray diffractometer (Bruker, Germany) with Cu-Kα radiation in the 2θ range of 0.5− 137°. UV−vis spectra were collected on a Shimadzu UV-3600 spectrophotometer (Shimadzu, Japan), and fluorescence spectra were recorded on an RF-5301 PC spectrophotometer (Shimadzu, Japan). High-performance liquid chromatographic measurements were carried out on an Elite P230-II high performance liquid chromatography (HPLC) (Dalian, China), and data were processed using W5100 chromatography workstation. ECL emission measurements were performed on an MPI-E ECL analyzer (Xi’an Remax Electronic Science and Technology Co. Ltd., Xi’an, China). The electrochemical measurements were carried on a CHI 660A electrochemical workstation (Shanghai Chenhua Equipment, China). A conventional three-electrode system was used to measure all of the ECL and electrochemical experiments. A modified glassy carbon electrode (GCE, 3 mm in diameter) served as the working electrode, a platinum wire electrode as the counter electrode, and an Ag/AgCl electrode as the reference electrode, except for electrochemical impedance spectroscopy (EIS) with a saturated calomel electrode. Synthesis of AuNPs. AuNPs of ca. 55 nm were synthesized as reported early.25 In brief, 3 mL of aqueous HAuCl4 solution (10 mM) was added to 100 mL of double-distilled water, and the solution was heated to 130 °C followed by addition of 0.6 mL of 1% sodium citrate under vigorous stirring. After continuous boiling for 30 min, the colloidal solution of synthesized AuNPs was allowed to cool down to room temperature and then centrifuged at 1500 rpm for 10 min to remove large NPs and finally stored at 4 °C for use. Preparation of Au@SiO2 NPs. To coat silica shells around AuNPs, the Au@SiO2 NPs were synthesized according to the method in the literature.26 The sequence of addition of TEOS and ammonia was important for uniform coating of silica around AuNPs. Method 1: 25 μL of MPTMS in ethanol (1 mM) was first added to 35 mL of the colloidal solution of AuNPs under vigorous stirring for 20 min, then different volumes of TEOS in ethanol (10 mM) were added to the colloidal solution to obtain Au@SiO2 NPs with different silica shell thicknesses, and finally 200 mL of ethanol and 3.6 mL of ammonia (28%) were added to the aqueous dispersion under stirring. The aqueous dispersion was slowly stirred at room temperature for 12 h and then allowed to age without agitation for 24 h. The aqueous dispersion of Au@SiO2 NPs was centrifuged at 10 000 rpm for 10 min, and the obtained Au@SiO2 NPs were washed with ethanol and double-distilled water four times followed by dispersing in 4 mL of double-distilled water. Method 2: ammonia and ethanol were added to the colloidal solution of AuNPs prior to the addition of TEOS under vigorous stirring. The subsequent procedures were the same as described in method 1. Preparation of Aminated Au@SiO2 NPs. The Au@SiO2 NPs with different silica shell thicknesses were functionalized with amino groups. Briefly, 1 μL of APTES was added to 2 mL of the colloidal solution of Au@SiO2 NPs, and the aqueous dispersion was shaken at room temperature for 5 h followed by heating at 50 °C for 1 h. Then, the resulting aminated Au@SiO2 (Au@SiO2-NH2) NPs were collected by centrifugation at 7000 rpm for 10 min and washed with ethanol and double-distilled water three times. Finally, the Au@ SiO2-NH2 NPs were dispersed in 2 mL of double-distilled water for use. Synthesis CdS QDs. TGA-coated CdS QDs were synthesized as reported previously.27 In brief, 250 μL of TGA was added to 50 mL of aqueous CdCl2 solution (10 mM) under a nitrogen atmosphere followed by dropwise addition of aqueous NaOH solution (1 M), and the pH was adjusted to 10−11 under stirring for 30 min. Then, 5 mL of aqueous Na2S solution (0.1 M) was added to the aqueous solution, and the solution was refluxed at 110 °C for 4 h under the nitrogen atmosphere. CdS QDs were eventually obtained as a precipitate by

addition of isopropyl alcohol (50 mL) to the solution. The precipitate was ultrasonically dispersed in double-distilled water, followed by centrifugation at 10 000 rpm for 10 min to collect the supernatant of CdS QDs. The aqueous solution of CdS QDs was transparent yellow and was stored in a refrigerator at 4 °C for use. To synthesize largesize CdS QDs, the volume of aqueous Na2S solution added was increased to 10 mL, and the reaction reflux time was extended to 8 h. Synthesis CdSe QDs. CdSe QDs were synthesized according to the literature.28 Selenium powder (0.1511 g, 2 mmol) and Na2SO3 (0.7563 g, 6 mmol) were added to 20 mL of double-distilled water, and the aqueous solution was refluxed at 70 °C for 24 h to give a clear NaSeSO3 solution (0.1 M). TGA (20 μL) was added to 20 mL of aqueous CdCl2 solution (5 mM) under a nitrogen atmosphere followed by first addition of aqueous NaOH solution (1 M), until the pH was adjusted to 10 and a clear solution was obtained and subsequent addition of 30 mL of double-distilled water under stirring for 30 min. Then, 0.5 mL of aqueous NaSeSO3 solution (0.1 M) was added to the above solution, and the solution was fluxed at 100 °C for 1 h under the nitrogen atmosphere to obtain a clear yellow solution of CdSe QDs. Isopropyl alcohol (50 mL) was added to the solution to precipitate CdSe QDs followed by centrifugation at 10 000 rpm for 10 min to remove the supernatant. Finally, the yellow precipitate of CdSe QDs was dispersed in 50 mL of double-distilled water and kept under 4 °C for use. Preparation of CdS QD-Decorated Au@SiO2-NH2 NPs. Two hundred microliters of the aqueous solution of CdS QDs were added to 250 μL of the colloidal solution of Au@SiO2-NH2 NPs, and the final volume of the colloidal solution amounted to 1000 μL by addition of double-distilled water. The aqueous dispersion was shaken at room temperature for at least 5 h to obtain CdS QD-decorated Au@SiO2-NH2 (denoted as Au@SiO2-NH2/CdS) NPs. Synthesis of AgNPs. AgNPs of ca. 60 nm were synthesized as reported previously.29 Briefly, 35 mg of AgNO3 was dissolved in 200 mL of double-distilled water, and the aqueous solution was heated to boiling. Sodium citrate (4 mL, 1%) was added to the boiling solution, and the solution was kept boiling for 1 h. The colloidal solution of AgNPs was centrifuged at 1500 rpm for 10 min to remove large NPsand then stored at 4 °C for use. Preparation of Ag@SiO2 NPs. The Ag@SiO2 NPs were prepared on the basis of the Stöber method.30 MPTMS (5 μL) in ethanol (1 mM) was dropwise added to 10 mL of the colloidal solution of AgNPs under vigorous stirring for 20 min, and then 40 mL of ethanol was added to the colloidal solution followed by addition of 0.7 mL of ammonia (28%) under stirring for 5 min. After that, 4.0 mL of TEOS in ethanol (10 mM) was added, and the aqueous dispersion was slowly stirred at room temperature for 12 h and then allowed to age without agitation for 24 h. Eventually, the colloidal solution of Ag@ SiO2 NPs was centrifuged at 7000 rpm for 10 min and washed with ethanol four times followed by dispersing in 10 mL of double-distilled water. Preparation of Hollow Silica NPs. Two milliliters of ammonia were dropwise added to 4 mL of the colloidal solution of Ag@SiO2 NPs under vigorous stirring for 20 min, and then the aqueous dispersion was stirred for 24 h. Finally, the colloidal solution of hollow silica (H-SiO2) NPs was centrifuged at 7000 rpm for 10 min and washed with ethanol four times followed by dispersing of H-SiO2 NPs in 10 mL of double-distilled water. Preparation of Aminated H-SiO2 NPs. One microliter of APTES was added to 2 mL of the colloidal solution of H-SiO2 NPs, and the aqueous dispersion was shaken at room temperature for 5 h followed by heating at 50 °C for 1 h. The amino-modified H-SiO2 (HSiO2-NH2) NPs were collected by centrifugation at 7000 rpm for 10 min and washed with ethanol and double-distilled water three times. Eventually, the H-SiO2-NH2 NPs were dispersed in 2 mL of doubledistilled water for use. Construction of ECL Platform. A GCE was polished with 1.0, 0.3, and 0.05 μm alumina slurries to a mirror-like gloss, then rinsed thoroughly with ethanol and double-distilled water for 30 s under ultrasound after each step, and finally dried under nitrogen stream. The colloidal solution of Au@SiO2-NH2/CdS NPs (10 μL) was C


Research Article


Figure 1. TEM images of Au@SiO2 NPs with different silica shell thicknesses (nm): (A) 0; (B) 3; (C) 6; (D) 7; (E) 8; (F) 9; (G) 10; (H) 14; (I) 17; (J) 19; (K) 23; (L) 30; (M) 34; (N) 38; (O) 41; (P) 44. Inset of (A) shows the HRTEM image of AuNPs and lattice spacings of neighboring fringes. Insets of (B−D) show the corresponding local magnifications. dropped onto the surface of the cleaned GCE by a pipette followed by drying under vacuum for at least 3 h (denoted as Au@SiO2-NH2/ CdS/GCE). Other modified electrodes were prepared using the similar procedures.

strongly depended on the addition sequence of TEOS and ammonia. Upon first addition of TEOS to the colloidal solution of AuNPs and subsequent addition of ammonia and ethanol, individual AuNPs were coated with uniform silica shells (Figure S1A). If the addition sequence was reversed, a few AuNPs gathered together and were coated with a silica shell (Figure S1B). This was because ammonia promoted the hydrolysis of TEOS. Upon first addition of ammonia, an increase in the ionic strength of solution, owing to the dissociation of ammonia, caused aggregation of individual AuNPs.31 The thickness of silica shell, ranging from 3 to 44 nm, could be well controlled by adjusting the amount of TEOS added (Figure 1B−P). From the TEM images in Figure 1, the dark and light areas of the Au@SiO2 NPs correspond to the Au cores and silica shells, respectively, due to the difference in electron density between them. After coating of silica shells, the XRD peaks of the Au cores decreased in intensity (Figure 2). The colloidal solution of AuNPs showed a very strong absorption band at 543 nm (Figure S2), due to the LSPR of AuNPs. The Au@SiO2 NPs underwent a continuous red shift in the LSPR band with the increase of silica shell thickness from 0 to 38 nm, due to the increase of local refractive index of silica around the Au cores.32 However, upon further increase of silica shell thickness, the LSPR band took a blue shift and a decrease in apparent intensity. This is because in this case the scattering of the AuNPs became dominant and their absorbances significantly increased at shorter wavelengths.33 In the case of core/shell NPs, all of these effects are explained by Mie theory.33 TGA-modified CdS QDs were synthesized and had an average size of 4 nm. Their HRTEM image shows a lattice spacing of 0.336 nm between neighboring fringes, corresponding to the (111) plane (Figure S3), which was consistent with the XRD patterns of the CdS QDs (Figure 2). The zeta potential of the TGA-modified CdS QDs was −27.1 mV, and the zeta potential of Au@SiO2 NPs was −20.1 mV (Figure S4). After the Au@SiO2 NPs were aminated with APTES, the zeta potential of the Au@SiO2-NH2 NPs was increased to 27.5

■

RESULTS AND DISCUSSION Characterizations of Au@SiO2-NH2/CdS Superstructures. Quasi-spherical AuNPs with an average diameter of 55 nm were first synthesized by reduction of HAuCl4 with sodium citrate, and the high-resolution transmission electron microscopy (HRTEM) image shows that the lattice spacings of 0.236 and 0.204 nm between neighboring fringes correspond to the (111) and (200) planes of the AuNPs, respectively (Figure 1A). The XRD patterns of the AuNPs showed nine diffraction peaks, indexed as (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes (Figure 2), which are well matched with the Au JCPDS card (no. 04-0784). The AuNPs were then modified with the coupling agent MPTMS to make their surfaces vitreophilic for convenient coating of complete silica shells. The morphology of the as-prepared Au@SiO2 NPs

Figure 2. XRD patterns of (a) AuNPs, (b) Au@SiO2-NH2 NPs, (c) Au@SiO2-NH2/CdS NPs, and (d) CdS QDs, together with standard XRD patterns of CdS and Au crystals for comparison. D


Research Article


depicted (Figure S6A). The Rct of a bare GCE was 99.0 Ω, and the Rct was decreased to 70.0 Ω for the AuNP modified GCE (Au/GCE) owing to the excellent conductivity of the AuNPs, but the Rct was increased to 835.8 Ω for the Au@SiO2-NH2 modified GCE (Au@SiO2-NH2/GCE) in the case of 44 nm silica shell because of the insulating property of silica. The corresponding cyclic voltammetry (CV) curves of these modified electrodes were also investigated (Figure S6B). Both anodic and cathodic peak currents were observed to diminish with the increase of silica shell thickness. The ECL responses of different modified electrodes in the 10 mM PBS solution (pH 7.4) containing 5 mM H2O2 as a coreactant are shown in Figure 5. The CdS QD-modified

mV. The N element was observed from the EDX mapping image of the Au@SiO2-NH2 NPs (Figure S5). It is clear that the Au@SiO2-NH2/CdS superstructures could be constructed through the electrostatic interaction as shown in Figure 3.

Figure 3. TEM image of Au@SiO2-NH2/CdS NPs. Insets show the HRTEM images of AuNPs and CdS QDs and the lattice spacings of corresponding neighboring fringes, respectively.

The TGA-modified CdS QDs showed an absorption edge at 480 nm and a maximum fluorescence emission at 610 nm (Figure 4). A maximum LSPR absorption was at 543 nm for

Figure 5. ECL curves of (a) CdS/GCE, (b) Au-CdS/GCE, (c) Au@ SiO2/CdS/GCE, (d) H-SiO2/CdS/GCE, (e) H-SiO2-NH2/CdS/ GCE, (f) Au@SiO2-NH2/CdS/GCE, and (g) Au@SiO2-QA/CdS/ GCE in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2. Inset shows the corresponding ECL curves in the absence of CdS QDs.

electrode (CdS/GCE) showed a cathodic ECL emission at −1.53 V (curve a in Figure 5), and the ECL intensity decreased upon addition of AuNPs to CdS/GCE (Au-CdS/ GCE) (curve b in Figure 5). The decrease of ECL response was attributed to the FRET between the CdS QDs and AuNPs,19 which could be further proved by the corresponding CV curves (Figure S7). A bare GCE showed almost no redox peak in the absence of H2O2, except for a very weak peak around −0.6 V due to the electrochemical reduction of dissolved oxygen,36 while an obvious reduction peak appeared in the presence of H2O2. CdS/GCE showed the cathodic reduction peak of CdS QDs at −1.40 V, besides the H2O2 reduction peak. However, the reduction peak of CdS QDs in the case of Au-CdS/GCE could not be observed as well as a strong reduction peak at −1.05 V (Figure S8), due to the electrochemical reduction of H2O2 promoted by AuNPs. These results indicate that the electron transfer between the CdS QDs and AuNPs took place within a small distance. The FRET between the CdS QDs and AuNPs could be effectively prohibited upon coating of silica shells on the surfaces of the AuNPs. The ECL emission of the modified electrode in the combination of CdS QDs and Au@SiO2 NPs (Au@SiO2/ CdS/GCE) was significantly enhanced to be ca. 15-fold stronger than that of CdS/GCE (curve c in Figure 5); moreover, the maximum ECL potential of Au@SiO2/CdS/ GCE shifted positively to −1.48 V relative to that of CdS/ GCE. The ECL emission of CdS QDs excited the LSPR of Au cores, and the LSPR-induced electromagnetic fields around the Au cores further enhanced the ECL emission of CdS QDs, and

Figure 4. UV−vis spectra of (a) CdS QDs, (b) AuNPs, and (c) Au@ SiO2-NH2 NPs (silica shell, 38 nm) and (d) fluorescence spectrum of CdS QDs (λex = 410 nm).

the AuNPs and shifted to 575 nm for the Au@SiO2-NH2 NPs with a silica shell of 38 nm (Figure S2). It is obvious that the LSPR absorption bands were overlapped with the absorption and emission bands of the CdS QDs, moreover, the extent of the overlap of the emission band of the CdS QDs with the LSPR band of the Au@SiO2-NH2 NPs was larger than that with the LSPR band of the AuNPs. Because there is a strong correlation between the fluorescence intensity of fluorophores and the degree of spectral overlap with LSPR bands of metallic NPs, emission maxima occur when the LSPR peaks are between the absorption and emission maxima of fluorophores.34 Dual Enhanced ECL of Au@SiO2-NH2/CdS Superstructures and ECL Enhancement Mechanisms. EIS was used to investigate the charge transfer properties between electrode surfaces and electrolytes. The semicircles at high frequencies correspond to charge transfer-limiting process, and their diameters are proportional to charge transfer resistance (Rct).35 In the presence of electroactive probes [Fe(CN)6]3−/4−, a series of GCEs were modified with the Au@ SiO2-NH2 NPs with the silica shells of 0−44 nm, and the semicircle diameter increased with the thickness of silica shells. The fitting of an equivalent circuit with the EIS data was E


Research Article


SiO2/GCE showed no redox peak in the absence of H2O2 and a weak reduction peak at −0.78 V in the presence of H2O2, and thus H-SiO2/CdS/GCE showed no reduction peak of CdS QDs as well as a reduction peak of H2O2 at −1.00 V, due to the insulating property of the H-SiO2 NPs. The silica shells were negative-charged and were further modified with amino groups, which facilitated adsorption of CdS QDs onto the surfaces of silica shells. After the H-SiO2 NPs were modified with APTES (H-SiO2-NH2), surprisingly, the ECL emission of H-SiO2-NH2/CdS/GCE was ca. 10- and 14-folds stronger than those of CdS/GCE and H-SiO2/CdS/ GCE, respectively, and the ECL potential at −1.53 V remained almost unchanged (curve e in Figure 5). In the absence of H2O2, CdS/GCE could also emit a weak ECL signal, while HSiO2-NH2/CdS/GCE emitted a weaker ECL signal than CdS/ GCE, although the identical amounts of CdS QDs were added for the two electrodes (Figure S12A). This comparison indicates that the modified amino species themselves could not act on coreactants for the ECL emission of CdS QDs. However, in the presence of H2O2, the significantly enhanced ECL emission was observed at −1.54 V from H-SiO2-NH2/ CdS/GCE (Figures 5 and S12B), which indicates that the modified amino species were involved in the ECL process in the assistance of H2O2 and ultimately promoted the enhanced ECL emission of CdS QDs. Few cases of the similar cathodic ECL enhancement by amino species have been reported,39−41 where the amino species were regarded to be oxidized to radical cations.39,41 It is obvious that a reasonable explanation or mechanism for this type of cathodic ECL enhancement is lacking so far. It is known that primary amines (R−CH2−NH2) can be oxidized to oximes (R−CHN−OH) by H2O2. The formed oximes were electrochemically reduced to generate radical anions (R−CHN−OH•−) followed by the rapid generation of free radicals (R−CHN•),42 due to the cleavage of weak N−O bonds.42,43 The generated free radicals (R− CHN•), similar to the OH• radicals from the electrochemical reduction of H2O2, combined with CdS radical anions (CdS•−) to produce excited state CdS intermediates (CdS*). The probable mechanisms of the ECL emission of the CdS−H2O2 system in the absence and presence of amino species are described in Chart S1. Furthermore, these ECL results were confirmed by corresponding CV curves (Figure 6B). The modified amino groups caused a shift in the reduction potential of coreactants from −0.78 V in the case of H-SiO2/GCE to −1.21 V in the case of H-SiO2-NH2/GCE, primarily due to the electrochemical reduction of the formed oximes, accompanied by a significant increase of peak current. H-SiO2-NH2/CdS/GCE also showed an increase in the reduction peak current of CdS QDs at −1.40 V. The difference in the reduction potential between CdS QDs and coreactants was decreased to 0.19 V in this case. It is the first to clarify that the amino species caused a decrease in the difference in the reduction potential between the emitters and coreactants for the increase of their peak currents, which facilitated the efficient combination of short-lived CdS radical anions (CdS•−) and free radicals (OH• and R−CHN•) generated in the small potential window to produce more excited state CdS intermediates (CdS*) for the enhancement of ECL emission (Chart S1). It is clear that the modified amino groups not only facilitated adsorption of CdS QDs onto the surfaces of silica shells but also enhanced ECL emission of CdS QDs. Furthermore, two types of ECL enhancement mechanisms were clearly proposed for the dual enhanced ECL of the Au@

thus the SEECL emission of Au@SiO2/CdS/GCE was achieved. The SEECL phenomenon can be explained that the LSPR-mediated far field effect due to the radiative decay of surface plasmons led to a prolonged optical path that could beneficially boost the excitation of electron−hole (e−−h+) pairs in CdS QDs,37,38 which ultimately resulted in a positive shift of the ECL potential. Seen from the corresponding CV curves shown in Figure 6A, the reduction peak of H2O2

Figure 6. (A) CV curves of (a) CdS/GCE, (b) Au@SiO2/GCE, and (c) Au@SiO2/CdS/GCE and (B) CV curves of (a) CdS/GCE, (b) SiO2-NH2/GCE, and (c) CdS-SiO2-NH2/GCE in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2.

appeared at −1.00 V in the case of Au@SiO2/GCE, and the reduction peak of CdS QDs shifted from −1.40 to −1.36 V and the peak current was significantly increased in the case of Au@SiO 2/CdS/GCE. The difference in the reduction potential between CdS QDs and H2O2 was 0.36 V. To reveal that the SEECL emission of Au@SiO2/CdS/GCE was related to the LSPR of the Au cores, H-SiO2 NPs were prepared from the Ag@SiO2 NPs. It is difficult to etch AuNPs, while it is easy to etch AgNPs. Ag@SiO2 NPs were first synthesized and then etched with ammonia. After etching, the LSPR band of Ag cores originally at 431 nm could not be observed (Figure S9), which indicates that the Ag cores were etched away. The SEM images before and after etching almost showed the identical morphologies (Figure S10), which indicates that the silica shells were well maintained even if the Ag cores were etched away. All of these results confirm that H-SiO2 NPs were successfully prepared. Furthermore, the ECL response of the modified electrode with the H-SiO2 NPs and CdS QDs (H-SiO2/CdS/GCE) was investigated (curve d in Figure 5), and no ECL enhancement could be available. This evidence strongly confirms that the SEECL as described above was contributed from the LSPR-induced electromagnetic fields of the Au cores. On the contrary, H-SiO2/CdS/GCE showed a slight decrease in the ECL intensity in comparison to CdS/ GCE, due to the insulating property of the H-SiO2 NPs. The ECL emission of Au@SiO2/CdS/GCE was ca. 21-fold stronger than that of H-SiO2/CdS/GCE (without Au cores). The corresponding CV curves of the modified electrodes (Figure S11) further supported the aforementioned ECL results. HF


Research Article


were oxidized by H2O2 followed by electrochemical reduction for chemical enhanced ECL. The synergistic effect of the two ultimately reached the achievement of dual enhanced ECL emission. Other important parameters and the amounts of CdS QDs and Au@SiO 2 -NH 2 NPs in the superstructures were optimized. First, the amount of Au@SiO2-NH2 NPs was fixed (250 μL), and then the volume of aqueous solution of CdS QDs was increased gradually from 0 to 350 μL. A maximum ECL emission was available when the volume of aqueous solution of CdS QDs was 200 μL (Figure S13). Excessive addition of CdS QDs could lower their excited state energies and led to prominent self-quenching.45 Similarly, upon increase of the volume of colloidal solution of Au@SiO2NH2 NPs from 50 to 450 μL, a maximum ECL emission was available at 250 μL (Figure S14). Under the optimal parameters, the ECL emission of Au@ SiO2-NH2/CdS/GCE was ca. 35-fold stronger than that of the counterpart (H-SiO2/CdS/GCE) (curve f in Figure 5), owing to the synergistic effect of the electromagnetic field enhancement induced by the LSPR of AuNPs and the chemical enhancement caused by the involvement of the amino species in the ECL process. To the best of our knowledge, the fold of ECL enhancement is the largest reported so far. It is worth noting that these NPs themselves, including AuNPs, H-SiO2, H-SiO2-NH2, Au@SiO2, and Au@SiO2-NH2 NPs, could not emit any ECL signal in the investigated potential range of −1.6 to 0 V (inset of Figure 5). It is demonstrated that the modified amino groups gave rise to additional ECL enhancement (called chemical enhancement herein). Thus, the Au@SiO2-NH2/CdS superstructures were successfully constructed for the dual enhanced ECL emission. To further support the chemical enhancement of modified amino groups and probable mechanism of chemical enhancement, Au@SiO 2 NPs were modified with quaternary ammonium groups (Au@SiO2-QA), and the ECL intensity of Au@SiO2-QA/CdS/GCE (curve g in Figure 5) was very comparable to that of Au@SiO2/CdS/GCE (electromagnetic field enhancement only) and much weaker than that of Au@ SiO2-NH2/CdS/GCE. It is demonstrated that the enhanced ECL emission of Au@SiO2-QA/CdS/GCE was contributed from electromagnetic field enhancement. Unlike the amino groups, the quaternary ammonium groups could not take part in chemical/electrochemical reactions for chemical enhancement of ECL. To gain deep insights into the generality of the dual enhanced ECL emission of the Au@SiO2-NH2/QD superstructures, the sizes of Au cores and CdS QDs and the use of other QDs were investigated in detail. The Au@SiO2 NPs with Au cores of different sizes (ca. 38, 55, and 75 nm) and silica shells of almost same thicknesses (22−23 nm) were synthesized for comparison (Figure 8). The ECL emission of Au@SiO2-NH2/CdS/GCE increased in intensity with the increase of Au core size (Figure 9). Larger Au cores gave rise to stronger electromagnetic field and better enhanced ECL emission of CdS QDs. It is clear that the ECL emission of the Au@SiO2-NH2/CdS superstructures can be further enhanced with the use of larger Au cores. The TGA-modified CdS QDs of ca. 12 nm (Figures S15 and S16) were used in the Au@SiO2-NH2/CdS superstructures (Au core, ca. 55 nm; silica shell, 38 nm), and the ECL emission of Au@SiO2-NH2/CdS/GCE was also enhanced in this case and was 9.7-fold stronger than that of the counterpart (CdS/

SiO2-NH2/CdS superstructures. One was the electromagnetic field enhancement induced by the LSPR of Au cores, and the other was the chemical enhancement caused by the modified amino groups, which were first oxidized by H2O2 and then electrochemically reduced. It is the first time to put forward the new concept of chemical enhanced ECL that was directly related to the participation of other chemicals, which caused a decrease in the difference in the redox potential between emitters and coreactants for the increase of their redox currents. The electromagnetic field enhancement of ECL was dominantly caused by the far field effect in comparison to that of SERS. The chemical enhancement of SERS results from the direct contact of analytes with nanostructured metallic surfaces, while the chemical enhancement of ECL has nothing with metallic NPs. Moreover, the chemical enhancement of ECL was well distinguished from the electromagnetic field enhancement of ECL, which was different from the coexistence of electromagnetic field enhancement and chemical enhancement in most SERS systems.4 Chemical enhanced ECL could be achieved independent of electromagnetic field enhancement, but chemical enhanced Raman scattering can hardly be achieved independent of electromagnetic field enhancement.7,44 Although the name of “chemical enhancement” of ECL herein is the same as that of SERS, their meanings in ECL and SERS are completely different. It is known that the electromagnetic field is related to the distance between the AuNPs and CdS QDs, that is, thickness of silica shells. The dependence of ECL intensity of Au@SiO2NH2/CdS/GCE on the thickness of silica shells was investigated in detail (Figure 7). The ECL intensity increased

Figure 7. (A) ECL curves of Au@SiO2-NH2/CdS/GCE with different silica shell thicknesses from 0 to 44 nm in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2. (B) ECL intensity as a function of silica shell thickness of Au@SiO2-NH2 NPs (n = 5).

with the increase of silica shell thickness and reached a maximum value when the silica shell was 38 nm and then decreased upon further increase of shell thickness. This was determined by the balance between FRET and electromagnetic field enhancement, dependent on the distance between CdS QDs and Au cores. The Au@SiO2-NH2/CdS superstructures with a silica shell of 38 nm were optimized for a maximum ECL emission. To the best of our knowledge, the optical path of maximum ECL enhancement is the longest reported so far. The Au cores with a size of ca. 55 nm were used to cause the LSPR-induced electromagnetic field enhancement for SEECL, and the modified amino groups on the surfaces of silica shells G


Research Article


Figure 8. TEM images of AuNPs [(A) 38 nm; (C) 55 nm; (E) 75 nm] and Au@SiO2 NPs [(B) Au core, 38 nm; silica shell, 23 nm. (D) Au core, 55 nm; silica shell, 23 nm. (F) Au core, 75 nm; silica shell, 22 nm].

simple and sensitive method for the determination of GSH. The ECL emission of Au@SiO2-NH2/CdS/GCE (Au core, ca. 55 nm; silica shell, 38 nm; CdS QD, ca. 4 nm) toward GSH of different concentrations was shown in Figure 10. The ECL

Figure 9. ECL curves of (a) CdS/GCE and (b−d) Au@SiO2-NH2/ CdS/GCE with different sizes of Au cores and thicknesses of silica shells in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2: (b) Au core, 38 nm; silica shell, 23 nm; (c) Au core, 55 nm; silica shell, 23 nm; (d) Au core, 75 nm; silica shell, 22 nm. Figure 10. (A) ECL curves of Au@SiO2-NH2/CdS/GCE (Au core, ca. 55 nm; silica shell, 38 nm; CdS QD, ca. 4 nm) toward GSH of different concentrations in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2. (B) Plot of decreased ECL intensity as a function of GSH concentration (n = 5).

GCE) (Figure S17). In addition, the TGA-modified CdSe QDs of ca. 6 nm (Figures S18 and S19) were used in the Au@SiO2NH2/CdSe superstructures (Au core, ca. 55 nm; silica shell, 38 nm). The ECL emission of Au@SiO2-NH2/CdSe/GCE was 8.5-fold stronger than that of the counterpart (CdSe/GCE) (Figure S20). These results demonstrate the generality of the dual enhanced ECL emission of the Au@SiO2-NH2/QD superstructures. Detection of GSH. To exploit the application of the dual enhanced ECL platform of the Au@SiO2-NH2/CdS superstructures, one of the analytes of recent interest, GSH, was detected. GSH is widely found in animals and plants (including vegetables and fruits) and plays an important role in the organism. GSH is the most abundant nonprotein thiol in the eukaryotic cells and plays a significant role in maintaining cellular redox homeostasis.46 GSH is one of the major endogenous antioxidant in cellular systems for defense against toxins and radicals,47 participating directly in the neutralization of free radicals and reactive oxygen compounds, as well as maintaining exogenous antioxidants such as vitamins C (ascorbic acid) and E in their reduced (active) forms.48 Abnormal levels of GSH can lead to cancer, aging, heart disease, psoriasis, leucocyte loss, AIDS, diabetes, cardiovascular disease, and other diseases.49 In plants, GSH is crucial for biotic and abiotic stress management. GSH is a pivotal component of the GSH−ascorbic acid cycle that decreases poisonous H2O2.50 Therefore, it is of significance to develop a

intensity decreased gradually with the increase of GSH concentration in the range of 0.05−7.0 mM (Figure 10A). To clearly understand the response mechanism of GSH, a series of control experiments were performed. In the absence of H2O2, the ECL intensity of Au@SiO2-NH2/CdS/GCE was decreased upon addition of GSH and then increased upon addition of H2O2 even to be stronger than the original one (Figure S21). In the presence of H2O2, the ECL intensity of Au@SiO 2 -NH2/CdS/GCE was rapidly decreased upon addition of GSH and then increased to some extent upon further addition of H2O2 but was much weaker than the original one (Figure S22). In the two cases, it is clear that GSH could quench the ECL emission of the Au@SiO2-NH2/CdS NPs. On one hand, GSH was readily oxidized to GSSG by the OH• and other radicals generated in the course of cathodic reduction of the CdS−H2O2 system, which resulted in the quenching of the ECL emission. On the other hand, the mixtures of GSH and H2O2 with different concentrations were analyzed in comparison with the individual components using HPLC (Figures S23 and S24). It is found that the mixture of GSH and H2O2 produced new species through the chemical reaction, and the new species was identified as GSSG in comparison with the pure component GSSG by HPLC. H


Research Article

ACS Applied Materials & Interfaces However, the content of the produced GSSG was very low under the same condition as that in the ECL measurements. In healthy cells and tissues, more than 90% of the total GSH pool is in the reduced form (GSH) and less than 10% exists in the oxidized form (GSSG), and the increase of GSSG/GSH ratio is considered to be an indicative of oxidative stress.51,52 The level of GSH in the intracellular matrix of tumor cells is elevated 2−3 orders of magnitude higher than that in the extracellular environment, up to ∼10 mM.53,54 It is clear that GSH could not only directly react with H2O2 but also capture the OH• radicals generated by the cathodic reduction of H2O2. Ultimately, the direct and indirect interactions of GSH with H2O2 resulted in the quenching of the ECL emission of the Au@SiO2-NH2/CdS NPs. To explore the effect of the produced GSSG on the detection of GSH, the ECL intensity of Au@SiO2-NH2/CdS/GCE toward GSSG in the presence of H2O2 was compared with that toward GSH (Figure S25). The ECL response toward GSSG was much smaller than that toward GSH. Regardless of the content of the produced GSSG, the contribution of GSSG to ECL response was already included in the ECL response toward GSH (Figure 10A). That is to say, the produced GSSG had no influence on the final detection of GSH. A plot of a decrease in the ECL intensity of the Au@SiO2NH2/CdS NPs as a function of different GSH concentrations is presented in Figure 10B. A good linear relationship in the range of 0.10−6.00 mM was obtained with a correlation coefficient of 0.9969, and the limit of detection was estimated to be 0.065 mM (S/N = 3). Moreover, the performance of the constructed ECL platform toward GSH was compared with those of other sensing platforms in the literature (Table S1). It is obvious that most of the detection ranges of GSH in the literature were lower than that of our ECL platform, even below the pM level. It is well known that the intracellular GSH concentrations are much higher than the pM level, especially the concentrations of GSH in tumor cells and some of vegetables are even at the mM level.53,54 One of the merits of our ECL platform was a broad linear concentration range of GSH under the physiological conditions of mM level, and dilution or even excessive dilution of biological samples will be avoided for the determination of GSH. It is known that tumor areas and inflammation tissues could generate abundant amount of H2O2 up to 50−100 μM.55−57 The ECL intensities of Au@SiO2-NH2/CdS/GCE toward GSH at the concentrations of H2O2 from 5 to 6 mM were compared (Figure S26). The ECL response toward GSH at the given concentration remained almost unchanged even if the concentration of H2O2 was increased by 1 mM, much higher than the H2O 2 concentrations in tumor areas and inflammation tissues. It is clear that the ECL response of the Au@SiO2-NH2/CdS/GCE platform toward GSH remains accurate in the cases of tumor areas and inflammation tissues. Selectivity of Constructed ECL Platform. The selectivity of the constructed Au@SiO2-NH2/CdS/GCE platform toward GSH was evaluated by measuring ECL emission in the presence of a number of interfering species (Figure 11). The ECL emission of the platform remained almost unchanged in the presence of alanine, glucose, fructose, K+, Ca+, and Cu2+ at 50 mM, in contrast to a remarkable decrease of ECL intensity in the presence of 5 mM GSH. Cysteine at 0.1 mM, dopamine, ascorbic acid, and uric acid at 0.5 mM had no significant influence on the ECL emission of the platform. The concentration of mitochondrial GSH was far higher than that

Figure 11. ECL responses of Au@SiO2-NH2/CdS/GCE toward 5 mM GSH, 0.1 mM cysteine, 50 mM alanine, 0.5 mM dopamine, 0.5 mM ascorbic acid, 0.5 mM uric acid, 50 mM glucose, 50 mM fructose, 50 mM KCl, 50 mM CaCl2, and 50 mM CuCl2, respectively, in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2.

of mitochondrial cysteine, protein thiols, and other endogenous antioxidants,58−60 and thus the interference from cysteine was minimal. It is obvious that the constructed ECL platform presented excellent selectivity toward GSH in biological samples. Stability and Reproducibility of Constructed ECL Platform. The stability and reproducibility of modified electrodes are also important factors for their practical applications. The constructed ECL platform showed excellent scan stability toward 3 mM GSH (pH 7.4) upon successive scan of 12 cycles (Figure 12). After 14-day storage at room

Figure 12. ECL responses of Au@SiO2-NH2/CdS/GCE (Au core, ca. 55 nm; silica shell, 38 nm; CdS QD, ca. 4 nm) toward 3 mM GSH in 10 mM PBS solution (pH 7.4) containing 5 mM H2O2 for 12 successive cycles.

temperature, the platform retained more than 96% of the original ECL response toward 3 mM GSH for the freshfabricated platform (Figure S27), which indicates that the constructed ECL platform had excellent storage stability. The reproducibility of the constructed ECL platform was also tested (Figure S28). Five independent modified electrodes had the relative standard deviation (RSD) of inter-assays of 2.5% toward GSH. The ECL platform showed the RSD of intraassays of 3.3% toward GSH after 10 tests. It is clear that the constructed ECL platform displayed excellent stability and reproducibility. I



■

Practical Application of Constructed ECL Platform in Human Serum. Sensing platforms for use in real samples not only widens their practical applications but also lays the foundation for point-of-care tests of diseases in the future. The constructed ECL platform was used to detect the level of GSH in human serum. The GSH concentrations were detected to be 0.72 and 0.35 mM for the serums of two volunteers. To further verify the accuracy of detection, the two serum samples were spiked with GSH at different concentrations. The recoveries of the spiked samples ranged from 97 to 101%, and the RSDs were less than 4% for 5 independent tests (Table 1). The

found (mM)

added (mM)

total (mM)

recovery (%)

RSD (%)

sample 1

0.72

sample 2

0.35

2.00 4.00 2.00 4.00

2.74 4.62 2.29 4.38

101.00 97.50 97.00 100.75

2.31 3.92 3.21 2.93

*E-mail: [email protected]. ORCID

Xuezhong Du: 0000-0003-2492-1085 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (no. 2017YFD0200306) and National Natural Science Foundation of China (no. 21872070).

■

REFERENCES

(1) Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (2) Albrecht, M. G.; Creighton, J. A. Anomalously Intense Raman Spectra of Pyridine at a Silver Electrode. J. Am. Chem. Soc. 1977, 99, 5215−5217. (3) Guerrini, L.; Graham, D. Molecularly-Mediated Assemblies of Plasmonic Nanoparticles for Surface-Enhanced Raman Spectroscopy Applications. Chem. Soc. Rev. 2012, 41, 7085−7107. (4) Zong, C.; Xu, M.; Xu, L.-J.; Wei, T.; Ma, X.; Zheng, X.-S.; Hu, R.; Ren, B. Surface-Enhanced Raman Spectroscopy for Bioanalysis: Reliability and Challenges. Chem. Rev. 2018, 118, 4946−4980. (5) Gersten, J.; Nitzan, A. Spectroscopic Properties of Molecules Interacting with Small Dielectric Particles. J. Chem. Phys. 1981, 75, 1139−1152. (6) Liu, N.; Prall, B. S.; Klimov, V. I. Hybrid Gold/Silica/ Nanocrystal-Quantum-Dot Superstructures: Synthesis and Analysis of Semiconductor−Metal Interactions. J. Am. Chem. Soc. 2006, 128, 15362−15363. (7) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392−395. (8) Guerrero, A. R.; Aroca, R. F. Surface-Enhanced Fluorescence with Shell-Isolated Nanoparticles (SHINEF). Angew. Chem., Int. Ed. 2010, 50, 665−668. (9) Zhang, X.; Kong, X.; Lv, Z.; Zhou, S.; Du, X. Bifunctional Quantum Dot-Decorated Ag@SiO2 Nanostructures for Simultaneous Immunoassays of Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF). J. Mater. Chem. B 2013, 1, 2198−2204. (10) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Electrochemistry and Electrogenerated Chemiluminescence from Silicon Nanocrystal Quantum Dots. Science 2002, 296, 1293−1297. (11) Richter, M. M. Electrochemiluminescence (ECL). Chem. Rev. 2004, 104, 3003−3036. (12) Miao, W. Electrogenerated Chemiluminescence and Its Biorelated Applications. Chem. Rev. 2008, 108, 2506−2553. (13) Liu, Z.; Qi, W.; Xu, G. Recent Advances in Electrochemiluminescence. Chem. Soc. Rev. 2015, 44, 3117−3142. (14) Zhang, J.; Gryczynski, Z.; Lakowicz, J. R. First Observation of Surface Plasmon-Coupled Electrochemiluminescence. Chem. Phys. Lett. 2004, 393, 483−487. (15) Chowdhury, M. H.; Malyn, S. N.; Aslan, K.; Lakowicz, J. R.; Geddes, C. D. First Observation of Surface Plasmon-Coupled Chemiluminescence (SPCC). Chem. Phys. Lett. 2007, 435, 114−118. (16) Shan, Y.; Xu, J.-J.; Chen, H.-Y. Distance-Dependent Quenching and Enhancing of Electrochemiluminescence from a CdS:Mn Nanocrystal Film by Au Nanoparticles for Highly Sensitive Detection of DNA. Chem. Commun. 2009, 8, 905−907. (17) Wang, J.; Shan, Y.; Zhao, W.-W.; Xu, J.-J.; Chen, H.-Y. Gold Nanoparticle Enhanced Electrochemiluminescence of CdS Thin Films

satisfactory recoveries show that the extent of possible interference by natural biomolecules in human serum samples can be ignored. Therefore, the constructed ECL platform has good accuracy in the detection of GSH in biological samples.

■

CONCLUSIONS In summary, the Au@SiO2-NH2/CdS superstructures were constructed for dual enhanced ECL. A maximum ECL emission of the Au@SiO2-NH2/CdS superstructures (Au core, ca. 55 nm) with a silica shell of 38 nm was 35-fold stronger than that of H-SiO2/CdS NPs in the presence of H2O2 as a coreactant. The fold of ECL enhancement is the largest and the optical path of maximum ECL enhancement is the longest reported so far. The ECL emission of the Au@ SiO2-NH2/CdS superstructures was further enhanced with the increase of Au core size. Two types of ECL enhancement mechanisms were clearly proposed for the dual enhanced ECL of the Au@SiO2-NH2/CdS superstructures. One was the electromagnetic field enhancement induced by the LSPR of Au cores, and the other was the chemical enhancement caused by modified amino groups, which were first oxidized by H2O2 and then electrochemically reduced. It was the first time to put forward the new concept of chemical enhanced ECL that was directly related to the participation of other chemicals, which caused a decrease in the difference in the redox potential between emitters and coreactants for the increase of their redox currents. The constructed ECL platform was demonstrated to have promising applications in highly sensitive determination of GSH, and the response mechanism of GSH was also explored.

■

AUTHOR INFORMATION

Corresponding Author

Table 1. Results for the Determination of GSH in Human Serum Samples human serum

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b14886. Additional TEM images, UV−vis spectra, CV curves, SEM images, ECL curves, probable mechanisms of the ECL emission, storage stability, and reproducibility of the sensing platform, comparison of the performances of sensing platforms, and HPLC chromatograms of the mixtures of GSH and H2O2 (PDF) J


Research Article

ACS Applied Materials & Interfaces for Ultrasensitive Thrombin Detection. Anal. Chem. 2011, 83, 4004− 4011. (18) Shan, Y.; Wu, H.; Xiong, S.; Wu, X.; Chu, P. K. Electrochemiluminescent Spin-Polarized Modulation by Magnetic Ions and Surface Plasmon Coupling. Angew. Chem., Int. Ed. 2015, 55, 2017−2021. (19) Wang, D.; Guo, L.; Huang, R.; Qiu, B.; Lin, Z.; Chen, G. Surface Enhanced Electrochemiluminescence of Ru(bpy)32+. Sci. Rep. 2015, 5, 7954. (20) Wang, D.; Li, Y.; Lin, Z.; Qiu, B.; Guo, L. Surface-Enhanced Electrochemiluminescence of Ru@SiO2 for Ultrasensitive Detection of Carcinoembryonic Antigen. Anal. Chem. 2015, 87, 5966−5972. (21) Hesari, M.; Swanick, K. N.; Lu, J.-S.; Whyte, R.; Wang, S.; Ding, Z. Highly Efficient Dual-Color Electrochemiluminescence from BODIPY-Capped PbS Nanocrystals. J. Am. Chem. Soc. 2015, 137, 11266−11269. (22) Liu, S.; Zhang, Q.; Zhang, L.; Gu, L.; Zou, G.; Bao, J.; Dai, Z. Electrochemiluminescence Tuned by Electron-Hole Recombination from Symmetry-Breaking in Wurtzite ZnSe. J. Am. Chem. Soc. 2016, 138, 1154−1157. (23) Wang, F.; Lin, J.; Zhao, T.; Hu, D.; Wu, T.; Liu, Y. Intrinsic ″Vacancy Point Defect″ Induced Electrochemiluminescence from Coreless Supertetrahedral Chalcogenide Nanocluster. J. Am. Chem. Soc. 2016, 138, 7718−7724. (24) Tan, X.; Zhang, B.; Zou, G. Electrochemistry and Electrochemiluminescence of Organometal Halide Perovskite Nanocrystals in Aqueous Medium. J. Am. Chem. Soc. 2017, 139, 8772−8776. (25) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nature 1973, 241, 20−22. (26) Ung, T.; Liz-Marzán, L. M.; Mulvaney, P. Optical Properties of Thin Films of Au@SiO2 Particles. J. Phys. Chem. B 2001, 105, 3441− 3452. (27) Zheng, J.; Huang, F.; Yin, S.; Wang, Y.; Lin, Z.; Wu, X.; Zhao, Y. Correlation between the Photoluminescence and Oriented Attachment Growth Mechanism of CdS Quantum Dots. J. Am. Chem. Soc. 2010, 132, 9528−9530. (28) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmüller, A.; Weller, H. ThiolCapping of CdTe Nanocrystals: An Alternative to Organometallic Synthetic Routes. J. Phys. Chem. B 2002, 106, 7177−7185. (29) Lee, P. C.; Meisel, D. Adsorption and Surface-Enhanced Raman of Dyes on Silver and Gold Sols. J. Phys. Chem. 1982, 86, 3391−3395. (30) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (31) Mine, E.; Yamada, A.; Kobayashi, Y.; Konno, M.; Liz-Marzán, L. M. Direct Coating of Gold Nanoparticles with Silica by a Seeded Polymerization Technique. J. Colloid Interface Sci. 2003, 264, 385− 390. (32) Liz-Marzán, L. M.; Philipse, A. P. Synthesis and Optical Properties of Gold-Labeled Silica Particles. J. Colloid Interface Sci. 1995, 176, 459−466. (33) Liz-Marzán, L. M.; Giersig, M.; Mulvaney, P. Synthesis of Nanosized Gold−Silica Core−Shell Particles. Langmuir 1996, 12, 4329−4335. (34) Chen, Y.; Munechika, K.; Ginger, D. S. Dependence of Fluorescence Intensity on the Spectral Overlap between Fluorophores and Plasmon Resonant Single Silver Nanoparticles. Nano Lett. 2007, 7, 690−696. (35) Zhang, X.; Du, X.; Huang, X.; Lv, Z. Creating ProteinImprinted Self-Assembled Monolayers with Multiple Binding Sites and Biocompatible Imprinted Cavities. J. Am. Chem. Soc. 2013, 135, 9248−9251. (36) Jiang, J.; Chen, D.; Du, X. Ratiometric Electrochemiluminescence Sensing Platform for Sensitive Glucose Detection Based on in situ Generation and Conversion of Coreactants. Sens. Actuators, B 2017, 251, 256−263.

(37) Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Yoshida, N.; Watanabe, T. A Plasmonic Photocatalyst Consisting of Silver Nanoparticles Embedded in Titanium Dioxide. J. Am. Chem. Soc. 2008, 130, 1676−1680. (38) Kumar, M. K.; Krishnamoorthy, S.; Tan, L. K.; Chiam, S. Y.; Tripathy, S.; Gao, H. Field Effects in Plasmonic Photocatalyst by Precise SiO2Thickness Control Using Atomic Layer Deposition. ACS Catal. 2011, 1, 300−308. (39) Jie, G.; Zhang, J.; Wang, D.; Cheng, C.; Chen, H.-Y.; Zhu, J.-J. Electrochemiluminescence Immunosensor Based on CdSe Nanocomposites. Anal. Chem. 2008, 80, 4033−4039. (40) Jie, G.-F.; Liu, P.; Zhang, S.-S. Highly Enhanced Electrochemiluminescence of Novel Gold/Silica/CdSe-CdS Nanostructures for Ultrasensitive Immunoassay of Protein Tumor Marker. Chem. Commun. 2010, 46, 1323−1325. (41) Yang, H.; Zhang, Y.; Li, L.; Zhang, L.; Lan, F.; Yu, J. Sudokulike Lab-on-Paper Cyto-Device with Dual Enhancement of Electrochemiluminescence Intermediates Strategy. Anal. Chem. 2017, 89, 7511−7519. (42) Soucaze-Guillous, B.; Lund, H.; Hashimoto, M.; Pettersson, L.; Barré, L.; Hammerich, O.; Søtofte, I.; Långström, B. Reduction of Oximes in Aprotic Media. Acta Chem. Scand. 1998, 52, 417−424. (43) Rodríguez, P.; Herrero, E.; Solla-Gullón, J.; Feliu, J. M.; Aldaz, A. Selective Electrocatalysis of Acetaldehyde Oxime Reduction on (111) Sites of Platinum Single Crystal Electrodes and Nanoparticles Surfaces. J. Solid State Electrochem. 2007, 12, 575−581. (44) Kong, X.; Yu, Q.; Lv, Z.; Du, X.; Vuorinen, T. Identification of Molecular Recognition of Langmuir-Blodgett Monolayers Using Surface-Enhanced Raman Scattering Spectroscopy. Chem. Commun. 2013, 49, 8680−8682. (45) Zhang, B.; Zhang, Y.; Mallapragada, S. K.; Clapp, A. R. Sensing Polymer/DNA Polyplex Dissociation Using Quantum Dot Fluorophores. ACS Nano 2010, 5, 129−138. (46) Ahn, Y.-H.; Lee, J.-S.; Chang, Y.-T. Combinatorial Rosamine Library and Application to in Vivo Glutathione Probe. J. Am. Chem. Soc. 2007, 129, 4510−4511. (47) Yin, J.; Kwon, Y.; Kim, D.; Lee, D.; Kim, G.; Hu, Y.; Ryu, J.-H.; Yoon, J. Cyanine-Based Fluorescent Probe for Highly Selective Detection of Glutathione in Cell Cultures and Live Mouse Tissues. J. Am. Chem. Soc. 2014, 136, 5351−5358. (48) Scholz, R. W.; Graham, K. S.; Gumpricht, E.; Reddy, C. C. Mechanism of Interaction of Vitamin E and Glutathione in the Protection against Membrane Lipid Peroxidation. Ann. N.Y. Acad. Sci. 1989, 570, 514−517. (49) Niu, L.-Y.; Guan, Y.-S.; Chen, Y.-Z.; Wu, L.-Z.; Tung, C.-H.; Yang, Q.-Z. BODIPY-Based Ratiometric Fluorescent Sensor for Highly Selective Detection of Glutathione over Cysteine and Homocysteine. J. Am. Chem. Soc. 2012, 134, 18928−18931. (50) Noctor, G.; Foyer, C. H. Ascorbate and Glutathione: Keeping Active Oxygen Under Control. Annu. Rev. Plant Physiol. Plant Mol. Biol. 1998, 49, 249−279. (51) Halprin, K. M.; Ohkawara, A. The Measurement of Glutathione in Human Epidermis Using Glutathione Reductase. J. Invest. Dermatol. 1967, 48, 149−152. (52) Pastore, A.; Piemonte, F.; Locatelli, M.; Lo Russo, A.; Tozzi, G.; Federici, G. Determination of Blood Total, Reduced, and Oxidized Glutathione in Pediatric Subjects. Clin. Chem. 2001, 47, 1467−1469. (53) Verma, A.; Simard, J. M.; Worrall, J. W. E.; Rotello, V. M. Tunable Reactivation of Nanoparticle-Inhibited β-Galactosidase by Glutathione at Intracellular Concentrations. J. Am. Chem. Soc. 2004, 126, 13987−13991. (54) Hong, R.; Han, G.; Fernandez, J. M.; Fernández, B.-j.; Forbes, N. S.; Rotello, V. M. Glutathione-Mediated Delivery and Release Using Monolayer Protected Nanoparticle Carriers. J. Am. Chem. Soc. 2006, 128, 1078−1079. (55) Szatrowski, T. P.; Nathan, C. F. Production of Large Amounts of Hydrogen Peroxide by Human Tumor Cells. Cancer Res 1991, 51, 794−798. K


Research Article

ACS Applied Materials & Interfaces (56) Chen, Q.; Liang, C.; Sun, X.; Chen, J.; Yang, Z.; Zhao, H.; Feng, L.; Liu, Z. H2O2-Responsive Liposomal Nanoprobe for Photoacoustic Inflammation Imaging and Tumor Theranostics via in Vivo Chromogenic Assay. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 5343−5348. (57) Lan, G.; Ni, K.; Xu, Z.; Veroneau, S. S.; Song, Y.; Lin, W. Nanoscale Metal-Organic Framework Overcomes Hypoxia for Photodynamic Therapy Primed Cancer Immunotherapy. J. Am. Chem. Soc. 2018, 140, 5670−5673. (58) Karp, D. R.; Shimooku, K.; Lipsky, P. E. Expression of γGlutamyl Transpeptidase Protects Ramos B Cells from Oxidationinduced Cell Death. J. Biol. Chem. 2000, 276, 3798−3804. (59) Yu, F.; Li, P.; Wang, B.; Han, K. Reversible Near-Infrared Fluorescent Probe Introducing Tellurium to Mimetic Glutathione Peroxidase for Monitoring the Redox Cycles between Peroxynitrite and Glutathione in Vivo. J. Am. Chem. Soc. 2013, 135, 7674−7680. (60) Fan, D.; Shang, C.; Gu, W.; Wang, E.; Dong, S. Introducing Ratiometric Fluorescence to MnO2 Nanosheet-Based Biosensing: A Simple, Label-Free Ratiometric Fluorescent Sensor Programmed by Cascade Logic Circuit for Ultrasensitive GSH Detection. ACS Appl. Mater. Interfaces 2017, 9, 25870−25877.

L


Dual Enhanced Electrochemiluminescence of Aminated Au@SiO2

Recommend Documents