the fluorescence detection of glutathione by ∙oh radicals...

Talanta 144 (2015) 551–558

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The fluorescence detection of glutathione by ∙OH radicals’ eliminationwith catalyst of MoS2/rGO under full spectrum visible light irradiation

Nan Zhang a,b, Weiguang Ma a,b, Dongxue Han a, Lingnan Wang a,b, Tongshun Wu a,n, li Niu a

a State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of AppliedChemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, Chinab University of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:Received 8 May 2015Received in revised form26 June 2015Accepted 2 July 2015Available online 5 July 2015

Keywords:Glutathione∙OH radicalMoS2/rGOFluorescent assay

x.doi.org/10.1016/j.talanta.2015.07.00340/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail address: [email protected] (T. Wu).

a b s t r a c t

In this study, a new method for the detection of glutathione (GSH) was designed based on the ∙OHradicals’ elimination system due to the reducing ability of GSH for the first time. Fluorescence methodwith terephthalic acid (TA) as the probe was employed for the quantification of ∙OH radicals’ productionand elimination. Experimental conditions of ∙OH radicals’ production were optimized in detail, and ∙OHradicals were found to be efficiently produced by the excellent catalysis performance of MoS2/rGO underfull spectrum visible light irradiation. The introduction of GSH make fluorescent intensity decrease due tothe elimination of ∙OH radicals. For the present fluorescence based GSH sensor, a wide detection range of60.0–700.0 mM and excellent selectivity have been achieved. Furthermore, it has been successfully em-ployed for the determination of GSH in commercial drug tablets and human serum.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Glutathione (γ-glutamyl-cysteinyl-glycine, GSH) is a ubiquitousnon-protein thiol compound which exists not only in mostmammalian tissues, but also in kinds of plants, including baker'syeast, wheat germ, tomatoes, pineapples, etc. [1–3]. GSH appearsto be a significant intracellular reduction–oxidation buffer since itcan be oxidized to the disulfide GSSG during the oxidative stress[4–7]. GSH is capable to protect cells from oxidative damage andtrap free radicals that can damage DNA and RNA, thereby de-creasing the possibility of kinds of diseases such as cancers, Par-kinson's disease, diabetes, as well as aging [8–13]. From this pointof view, GSH as an antioxidant plays an essential role in the healthof organisms. Thus, the detection of GSH contributes a lot to thenutriology, pharmacy and clinical significance.

In recent years, fluorescent approaches based on differentmechanisms have been designed and investigated. Among them,those based on interactions between GSH and metal ions (Hg2þ ,Au, Cu2þ , etc.) have attracted tremendous interests [14–22].Nevertheless, the interactions are actually between metal ions andthiol group of GSH, which led to the disadvantages for the unablediscrimination between GSH and other biological thiols, such ascysteine (Cys) and homocysteine (HCy) [16,20,21]. Moreover, se-lective detection of GSH over Cys and HCy is essential for practical

applications when the structurally related interferential biothiolsare presented. Therefore, great efforts have been paid and corre-sponding strategies have been reported [17,23–25]. On the otherhand, the employment of heavy metal is unfriendly to the en-vironment as well. To further solve these problems simulta-neously, a large amount of fluorescent probes have been designedand reported, which are usually organic macromolecules, such asnaphthalimide-based probe [26], cationic squaraine dye basedprobe [27], octadentate macrocyclic Tb(III) cyclen conjugate pos-sessing a maleimide functionality [28], 1,1-dimethyl-2,5-bis(meso-formylphenyl) -3,4-diphenylsilole (DMBFDPS) [24], 4-amino-2,2,6,6-tetramethylpiperidine oxide (AT)-functionalized CdTequantum dots (QDs-AT) [29], 5,5′-((4,4′-(2,2′-diselanediylbis(benzoyl))bis(piperazine-1,1′-carbon-othioyl))bis(azanediyl))bis(2-(6-hydroxy-3-oxo-3H-xanthen-9-yl) benzoic acid) (FSeSeF) [30],etc. However, the complicated design and synthesis procedures ofthese intricate probes limit further practical application. Con-cerning the above mentioned problems, new strategies with moresimplicity and convenience are expected. Reducing ability, one ofthe most notable properties of GSH, should be utilized for thedetection of GSH, just as methods based on reduction of MnO2 orH2O2 by GSH [23,31], as well as oxidization of GSH by electrons onmodified electrodes as reported [32–34]. As is well known, ∙OHradical is an active reactive oxygen species with remarkable oxi-dative capacity, which is expected as the promising candidate forthe detection of GSH. Moreover, ∙OH radicals can be producedeasily and efficiently via the photocatalysis through varioussemiconductor materials such as TiO2, ZnO, etc. [35,36]. Compared

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N. Zhang et al. / Talanta 144 (2015) 551–558552

with the application of ultraviolet irradiation (UV) in fluorescentassay for bioagents [32,35], the employment of visible light sourcewill lead to energy saving, less damage to DNA and cells [37,38], aswell as blank background.

In this investigation, MoS2 grown on reduced graphene oxide(rGO) nanocomposite (MoS2/rGO) was applied as the photo-catalyst to efficiently produce ∙OH radicals under full spectrumvisible light irradiation. MoS2 is known as a layered structuresemiconductor with a narrow band gap of about 1.8 eV [39–43],while rGO was applied as both the substrate of MoS2 and theelectron transfer channel to prolong the life of the separatedelectrons and holes [44,45]. Till now, such a composite is firstutilized for GSH fluorescence determination based on ∙OH radicals’generation and elimination under visible light irradiation. Duringthe detection process, terephthalic acid (TA) was applied as thefluorescent probe [35,46–48]. By applying this strategy, an ex-cellent leaner response between the decrease of fluorescent in-tensity and quantity of GSH was obtained. The advantages alsoinvolve rapid detection response, wide detection range and highselectivity. Furthermore, based on this sensor, GSH determinationsupon practical samples of commercial drug tablets and humanserum have been successfully applied. The advisable feasibility inpractical applications makes this fluorescent sensor a brilliantcandidate for GSH sensing.

2. Experimental section

2.1. Reagent

Graphite powder (7782-42-5) was obtained from (chemicallypure, CP) Sinopharm Chemical Reagent Co., Ltd. (NH4)2MoS4(15060-55-6), N2H4 �H2O (7803-57-8) and L-Glutathione (GSH,27025-41-8) were purchased from Sigma-Aldrich. CommercialTiO2 (P25, 13463-67-7) was got from Degussa, Germany. TA (100-21-0) was obtained from Alfa. Uric acid (69-93-2) and ascorbic acid(36431-82-0) were bought from Fluka. L-Histidine (His, 71-00-1),L-Proline (Pro, 147-85-3), L-Lysine (Lys, 56-87-1) and L-Threonine(Thr, 6028-28-0) were purchased from DingGuo Biotech. (Beijing,China). β-Alanine (Ala, 107-95-9) was obtained from Kayon Bio-tech. (Shanghai, China). L-Cysteine (Cys, 52-90-4) and D-fructose(57-48-7) were purchased from Huishi Biochemical Reagent(Shanghai, China). DL-malic acid (617-48-1) was obtained from YiliChemicals, (Beijing, China). D (þ)-Glucose (50-99-7) was pur-chased from Biomics Biotech (Nantong, China). N, N-di-methylformamide (DMF, 68-12-2) and all the other inorganic re-agents were bought from Beijing Chemicals Co., China. Unlessotherwise stated, reagents were of analytical grade and used asreceived. All aqueous solutions were prepared with doubly dis-tilled water (DI water) from a Millipore system (418 MΩ cm).

2.2. Instruments

Images of scanning electron microscope (SEM) were obtainedon an XL30 ESEM FEG field emission scanning electron micro-scope. Transmission electron microscopy (TEM) images were takenon a Hitachi-600 TEM at an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) energy dispersive X-ray (EDX) spectrumand scanning transmission electron microscopy (STEM) and ele-mental mapping images were all carried out on a Fei Tecnai G2 F20S-TWIN transmission electron microscope operating at 200 kV.Raman spectra were obtained on a Renishaw Raman system model1000 spectrometer. X-Ray diffraction pattern (XRD) was obtainedwith a Siemens D5005 diffractometer with Cu Kα radiation, andwas applied to investigate the crystallographic structure of the as-fabricated products. X-Ray photoelectron spectroscopy (XPS)

analysis was carried out on an ESCALAB MKII X-ray photoelectronspectrometer with Al Ka X-ray radiation as the X-ray source forexcitation.

2.3. Preparation of MoS2/rGO

Graphene oxide (GO) was prepared using a modified Hummersmethod [49]. Graphite powder (0.032 g) was poured into 50 mL ofconcentrated H2SO4 under an ice-water bath. Then, 0.5 g NaNO3and 3 g KMnO4 were gradually added. The mixture was stirred for2 h then diluted with distilled water. After that, 10 mL 30% H2O2was added to the solution until the color of the mixture changed tobrilliant yellow. The slurry was poured into water. Then graphiteoxide was exfoliated with ultrasonic treatment. The product wasisolated by filtration, rinsed thoroughly with DI water and thendried in vacuum.

For the preparation of MoS2/rGO, 6 mg GO was dispersed wellin 20 mL DMF by sonication. 13.2 mg (NH4)2MoS4 was introducedto the above solution and the mixture was sonicated for severalminutes until a clear and homogeneous solution was achieved.After that, 60 μL N2H4�H2O was dropped into the system. Thereaction solution was further sonicated for 30 min before trans-ferring to a 40 mL Teflon-lined autoclave. The autoclave was he-ated at 200 °C for 10 h and then cooled to room temperaturenaturally. Product was collected by centrifugation, thoroughlywashed with water and ethanol alternately for several times, andthen dried by a lyophilizer overnight. Pristine MoS2 and pristinerGO were synthesized as the above procedures without adding GOor (NH4)2MoS4 respectively.

2.4. Sample preparation

The standard addition method was used for the determinationof GSH in practical samples. Fresh human serum and commercialtablets of reduced glutathione (Chongqing Yaoyou PharmaceuticalCo., Ltd.) were obtained from the local hospital. The GSH spikeddiluted human serum samples were prepared by adding differentamounts of GSH into the as-prepared diluted human serum andmixed well. The sample solution of commercial drug was preparedby completely grinding and homogenizing of the reduced glu-tathione tablets. 10 mg tablet powder was accurately weighed anddissolved in 100 mL water with constant sonication. The samplestock solution (0.1 mg/ml) was obtained after filtering the sus-pension with an ordinary filter paper. The GSH spiked diluted ta-blet samples were prepared by adding different amounts of GSHinto the as-prepared diluted tablet samples and mixed well.

2.5. Fluorescence measurement

Phosphate buffer solution (PBS) of various pH values (5.8, 6.4,7.4) were prepared by the stock standard solutions of NaH2PO4 andNa2HPO4. Buffer solution of pH¼9.0 was prepared by the stocksolutions of Na2B4O7 and H3BO3. Bicarbonate buffer solution (BBS,pH¼10.8) was prepared with stock solutions of Na2CO3 andNa2HCO3. Each buffer solution with a final concentration of 50 mMcontaining 10 mM NaCl was prepared for further use. TA wasdissolved in a minimum volume of 1 M NaOH aqueous solution,treated by sonication until complete dissolution and adjusted withbuffer solution to a concentration of 1 mM TA solution.

Photocatalysis process was performed as the following proce-dure: MoS2/rGO composite was added into the TA solution with orwithout GSH and the dispersion was kept stirring under the fullspectrum visible light illumination of xenon arc lamp (CHF-XM35,Beijing Trusttech Co. Ltd., China) equipped with a UV cutoff filter(λ4420 nm). Monochromatic filters of different wavelengths of420, 500, 550, 600 and 650 nm were employed to investigate the

N. Zhang et al. / Talanta 144 (2015) 551–558 553

effect of wavelength on the production of ∙OH radicals. Light in-tensity meter (CEL-NP2000, Beijing CEAuLight Co. Ltd., China) wasemployed to exactly identify the illumination intensity. During theillumination, the reaction system was purged with nitrogen. Afterirradiation, the clarified solution was obtained by a syringe filterimmediately and then transferred into the quartz cell for fluores-cence measurement at once. Fluorescence spectra were measuredwith an excitation wavelength of 315 nm by a Hitachi F-4600spectrometer.

3. Results and discussions

3.1. Characterization of MoS2/rGO

The morphologies of MoS2/rGO, rGO and MoS2 are shown inFig. 1 and Fig. S1, respectively. Compared with the TEM image ofrGO blank substrate (Fig. 1A), the TEM image of MoS2/rGO (Fig. 1B)illustrates the successful decoration of MoS2 on rGO. It can beobserved from the high-magnification TEM image of MoS2/rGO(Fig. 1C) that MoS2 are uniformly dispersed on rGO. Both thecontrasts between SEM images of MoS2 and MoS2/rGO and theirTEM images (Fig. S1A vs. Fig. S1B, Fig. S1C vs. Fig. S1D) indicate

Fig. 1. TEM image of rGO (A); TEM (B) and high-magnification TEM images of MoS2/rGOmapping of S and Mo (E).

that rGO played a significant role as an excellent substrate materialproviding preferable spread ability, which ensures a larger ex-posure of abundant photocatalytic active edge sites of MoS2 byunfolding the frizzy pristine MoS2 [50,51]. HRTEM image (Fig. 1D)of MoS2/rGO exhibits parallel lines, which is corresponding to thelayers of MoS2 with an interlayer distance of 0.62 nm ascribed tothe (002) plane of hexagonal MoS2. The elemental mapping ima-ges of S and Mo corresponding to STEM image (Fig. 1E) furtherexhibit the homogeneous distribution of S and Mo in the com-posite, which reveals that MoS2 has been uniformly decorated ongraphene sheet. This complementarily points out that GO is asuitable substrate for the nucleation and subsequent growth ofMoS2, which should attribute to the interactions between func-tional groups on GO sheets and Mo precursors in the suitablesolution environment [52].

EDX spectrum of MoS2/rGO (Fig. S2) testifies the presence of C,O, Mo and S elements. The chemical state of MoS2 in MoS2/rGOwas further confirmed through XPS spectra. As shown in Fig. 2A,the binding energies of Mo 3d5/2 and Mo 3d3/2 peaks at 228.6and 231.8 eV suggest the dominance of Mo4þ in the product. InFig. 2B, the peaks at binding energies of 161.6 and 162.6 eV shouldbe assigned to S 2p3/2 and S 2p1/2, respectively, which indicatethat S2� is the domain states. Moreover, the XPS spectrum of C 1s

(C); HRTEM image of MoS2/rGO (D); STEM image and the corresponding elemental

Fig. 2. XPS of MoS2/rGO composite: high resolution Mo 3d spectrum (A); S 2p spectrum (B); C 1s spectrum (C); XRD pattern of MoS2/rGO (D) and Raman spectra of MoS2,rGO, and MoS2/rGO (E).

Scheme 1. Illustration of GSH detection based on MoS2/rGO photocatalysis system.


further confirms the reduction of GO to rGO (Fig. 2C).The structure of MoS2/rGO was further characterized by XRD

pattern. As shown in Fig. 2D, characteristic peaks at 2θ¼32.9,56.7° corresponding to the (100) and (110) planes of MoS2 confirmthe hexagonal phase of MoS2. And the broad peak centered at23.7° is ascribed to rGO according to the previous reports [53–56].Fig. 2E shows the Raman spectra of MoS2, rGO, and the MoS2/rGO.Peaks at 373 and 400 cm�1, corresponding to the A1g and E2g, arethe characteristic peaks of MoS2. While D band (1370 cm�1) and Gband (1600 cm�1) in the higher wave number region are thecharacteristic peaks of rGO.

3.2. Optimization of experimental conditions

In this work, for the first time GSH is detected by the strategy ofeliminating ∙OH radicals produced by MoS2/rGO under visible lightirradiation. The proposed operating mechanism of this strategysystem is represented in Scheme 1. Under visible light irradiation,the electron in MoS2 will transfer from valence band (VB) toconductive band (CB), leaving the created holes (hþ) to furtheroxidize hydroxide ions or water molecules and thereby producinglots of ∙OH radicals at the photocatalyst/solution interfaces. TA isused as the probe to react with ∙OH radicals to form the fluor-escent compound 2-hydroxyterephthalic acid (HTA, fluorescenceresponse at 425 nm) [35,46–48], which should be employed forthe quantification of ∙OH radicals by fluorescent evaluation. WhenGSH is introduced into the system, it is able to capture and elim-inate ∙OH radicals as a competitor to TA, which will decrease thefluorescent response of HTA. According to the reaction process,several factors including concentration of photocatalyst, irradia-tion duration, light intensity, wavelength and pH value have beenthoroughly investigated.

Fig. 3A exhibits the influence of the concentration of MoS2/rGOon ∙OH radicals’ production. It is observed that more ∙OH radicals

produced with higher concentration of catalyst, as the fluores-cence intensity increased with the concentration from 40 to640 mg/mL (inset of Fig. 3A). However, when the concentration waslarger than 240 mg/mL, the increase apparently slowed down.Therefore, during the experiments, 240 mg/mL was finally selectedand regarded as the optimum concentration of the catalyst.

Illumination time is also an important factor which will sig-nificantly affect the generation of ∙OH radicals. Generally, in-creased fluorescent response was achieved with the extending ofthe illumination time (Fig. 3B). As shown in the inset of Fig. 3B, theamount of ∙OH radicals keeps increasing from 1 to 40 min irra-diation, which will achieve a quite high intensity. While the in-crease of the signal response after 5 min gradually slows down. Inthe case, 5 min was considered as the optimum illumination time,which has preferably reduced the time consuming based on GSHdetermination comparing with previous literatures [16,57,58].

Furthermore, study on light intensity which was controlled

Fig. 3. Fluorescence responses of the MoS2/rGO suspension with increasing concentration of MoS2/rGO (inset: the relationship between fluorescence intensity and con-centration of the MoS2/rGO) (A); different illumination time (inset: the relationship between fluorescence intensity and illumination time) (B); increasing light intensity(inset: the relationship between fluorescence intensity and light intensity) (C) and fluorescence responses of rGO, MoS2, P25 and MoS2/rGO in TA solution under optimumexperimental conditions (D). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)


from 75 to 300 mW/cm2 according to the light intensity meter hasbeen carried out. As is shown in Fig. 3C, the fluorescence responseis proved to be in leaner relationship with light intensity. Ac-cording to the moderate ∙OH radicals’ preparation and distinctsignal response, 300 mW/cm2 of the light intensity was adequatein this system.

In addition, the effect of wavelength on the ∙OH radicals’ pro-duction was also investigated from 420 to 650 nm. As shown inFig. S3, visible light with the same light intensity but differentwavelengths exhibit almost the same fluorescence expression. Theeffective full spectrum visible light excitation of MoS2/rGO shouldbe ascribed to the inherent narrow bandgap of this photocatalyst.Although UV as an effective excitation light source which canprovide higher energy, its disadvantages to physiological en-vironment and the interference to the fluorescence backgroundcan not be neglected. As shown in Fig. S4, almost no interferencetakes place when the UV cutoff filter was employed (a and c).While obvious fluorescence responses of TA containing GSH(b) and TA (d) appear after illuminated without UV cutoff filter. Forthe above reasons, full spectrum visible light was chosen as thelight source in our system.

Finally, the effect of system pH value was investigated from5.8 to 10.8. As shown in Fig. S5, when the pH vary from 5.8 (blackline) to 6.4 (red line), the fluorescence intensities are relativelysmall and diminutive change can be observed. While the in-tensities increase dramatically when the pH was adjusted in therange of 7.4–10.8. Generally, the production of ∙OH radicals basedon photocatalysis should be in accordance with the following

reactions [59–63]:MoS2þhν-hþþe�H2O2HþþOH�OH�þhþ-�OHhþþH2O-�OHþHþApparently, different pH will lead to distinguished reaction

equilibrium, which will subsequently cause different quantity of∙OH radicals. Though the generation of �OH was found to be de-pendent on pH, according to the considerable fluorescent intensityin the biological neutral environment, physiological pH 7.4 waschosen to be the optimum pH in this system.

In summary, the experimental conditions were finally opti-mized and listed as following: the catalyst concentration was240 mg/mL; the illumination time was 5 min; the light intensitywas 300 mW/cm2; the light source was full spectrum visible light,and the pH value was 7.4.

Under the above optimum conditions, excellent reproducibilityof ∙OH radicals’ production based on MoS2/rGO photocatalysis wasachieved, since almost the same fluorescent signals were obtainedthrough five separated experiments (Fig. S6). However, comparedwith MoS2/rGO composite, the commercial P25 (red line), pristineMoS2 (cyan line) or rGO (blue line) demonstrated unsatisfactorycapability towards the generation of ∙OH radicals, which is shownin Fig. 3D. Although P25 is considered as a nice commercial pho-tocatalyst, the instinct wide band gap restricts the efficiency of∙OH radicals’ production under visible light irradiation. In spite ofthe narrow band gap of MoS2, for pristine MoS2, the photo-generated electrons and holes are easy to recombine, which also

Fig. 4. Fluorescence spectra of MoS2/rGO suspension with different concentrations of GSH (0, 60, 120, 190, 260, 370, 470, 600, 700, 830, 1000 and 1400 μM) (A); therelationship of fluorescence intensity decrease and the concentration of GSH in the range of 60–1400 μM (B).

Fig. 5. The fluorescence responses of the MoS2/rGO system to GSH (500 μM) andkinds of disruptors such as amino acids (5 mM, including Lys, Cys, Ala, Pro, Thr,His), malic acid (5 mM), glucose (5 mM), fructose (5 mM), ascorbic acid (500 μM),uric acid (500 μM), some inorganic anions and cations (0.5 M, including Naþ ,Mg2þ , Kþ , Cl� , SO42�) when they coexist with GSH (500 μM).


results in low productivity of ∙OH radicals. rGO with hardly ob-served fluorescent response in this system, is known for its perfecttwo dimensional structure, which has been widely applied as anelectron reservoir to inhibit the recombination of electron–holepairs, and therefore enhance photoactivity of semiconductors. Inaddition, the abundant functional groups on GO helped MoS2 toanchor and grow tightly onto the rGO sheet. rGO as an excellentsubstrate effectively prevented the agglomeration of MoS2 andmade the active edges sufficiently exposed outside, which hasgreatly improved the photocatalytic efficiency of the semi-conductor. Consequently, the hybridization of MoS2 and rGOmakes ∙OH radicals’ production efficiency dramatically improvedunder visible light illumination. In general, the hybrid of MoS2/rGOis considered as a preeminent material for the application ofphotocatalysis under visible light illumination.

3.3. Assay of GSH

With the introduction of GSH, the fluorescence intensity wasobserved to decrease as expected (Fig. 4A), since GSH is capable toeliminate ∙OH radicals and thus reduce the production of thefluorescent HTA molecules. Optimum experimental conditionswere applied throughout the assay of GSH and excellent re-producibility was achieved. Calibration curve was then con-structed by measuring the fluorescent responses of standard GSHsolutions at different concentrations in triplicate (n¼3). As shownin Fig. 4B, the linear relationship between decrease of fluorescenceintensity at 425 nm (ΔF) and the concentrations of GSH (mM) witha wide detection range of 60.0–700.0 μM was obtained, whichfollows the equation ΔF¼5.9048þ0.1645C (R¼0.9973). The de-tection limit of the proposed method, calculated following the 3SB/S International Union of Pure and Applied Chemistry (IUPAC) cri-teria, is 25.0 μM [64]. Compared with some previously reportedfluorescence based assay of GSH, this system shows a larger de-tection range [65,66].

3.4. Selectivity

In order to evaluate the selectivity of this strategy for the de-termination of GSH, it is essential to examine the fluorescent re-sponses of some other biomolecules which might usually coexistwith GSH in practical samples. The investigation was operatedunder optimized experimental conditions and the results areshown in Fig. 5. Herein, the relativeΔF (GSH) was set to 100%, andthe relative ΔF (disrupt species) was calculated following theformula below.

RelativeΔF (disrupt species)¼[ΔF (GSHþdisrupt species)�ΔF

(GSH)]/ΔF (GSH).It indicates that none notable interferences occur with the co-

existence of 500 mM GSH and kinds of disrupt species, includingvarious amino acids (5 mM, fundamental composition of organ-ism), glucose and fructose (5 mM, weakly reducing bioagents),malic acid and ascorbic acid (5 mM and 500 μM, additive of foodand medicine), uric acid (500 μM, essential metabolite in organ-isms) and some inorganic anions and cations (0.5 M, electrolytes).This interference investigation reveals that this photocatalysisbased fluorescence GSH determination demonstrated excellentselectivity and exhibited favorable anti-interference property to-wards common disruptors in biological environment. It is worthmentioning in particular that, Cys as a kind of universal amino acidis always coexist in the biological system, which is usually in-volved into the detection of GSH due to the same thiol group.However, in our system, even the concentration of Cys was 10times of the analyte, almost no disturbance was observed. Such anadvisable selectivity should be ascribed to the advantageous assaymechanism. The fluorescence examination of ∙OH radicals’ elim-ination based on the antioxidant capability of GSH rather than itsthiol group, which can efficiently avoid interference of moleculeswith similar structure.

3.5. Application in practical samples

To further demonstrate the application of this new GSH sensor,the recovery and relative standard deviation (RSD) were tested by

Table 1Determination of GSH in practical samples of commercial drug tablets and humanserum (n¼3).

Sample Added (μM) Detected (μM) Recovery (%) RSD (%)

160.0 168.4 105.3 1.04Tablets 260.0 257.3 99.0 0.99

360.0 353.1 98.1 1.25

Human serum 100.0 97.2 97.2 2.31150.0 157.8 105.2 3.02200.0 213.5 106.7 1.85


a standard addition method for the detection of GSH in practicalsamples. First, commercial reduced glutathione drug tablets sam-ple was chosen as one example. Similar detection procedures wereconducted with the mixed solution of GSH and the practicalsamples under optimized experimental conditions. As exhibited inTable 1, the recovery of commercial tablets was obtained in therange of 98.1–105.3%. Furthermore, a more complicated system,human serum, was applied as another practical sample, whichgenerally contains proteins, hormones, glucose, and other biolo-gical substances. The recovery of GSH in human serum was testedand calculated to be 97.2–106.7% (shown in Table 1), which wasquite appropriate compared with other methods for GSH de-termination in serum as previously reported [18,19,21,27]. It in-dicates that the detection of GSH based on this new strategy canbe satisfactorily achieved in practical applications either for com-mercial drugs or human serum samples.

4. Conclusion

In this work, a novel approach for the detection of GSH hasbeen designed and thoroughly investigated. For the first time,examination of GSH was realized by the elimination of ∙OH radi-cals and monitored by fluorescent technique with TA as probe.Under full spectrum visible light irradiation, ∙OH radicals are foundto be produced with high efficiency by the excellent photocatalyticperformance of MoS2/rGO composite. The experimental conditionsupon photocatalysis of MoS2/rGO were then optimized in detail.And a wide detection range of GSH concentration from 60.0 to700.0 μM was achieved. Most importantly, the fluorescence re-sponse of GSH is remarkably specific in the presence of commondisrupt species, including some amino acids and kinds of bioa-gents, which will well meet the approval of selective requirementsfor biochemistry and clinical analysis application. Furthermore,GSH detection in practical samples of commercial reduced glu-tathione tablets and human serum have also been successfullyperformed, which demonstrate the favorable feasibility of this newstrategy in the practical applications.

Acknowledgment

The authors are most grateful to the NSFC, China (Nos.21205112, 21225524, 21475122 and 21127006), the Department ofScience and Techniques of Jilin Province (Nos. 20120308,201215091 and SYHZ0006) and Chinese Academy of Sciences(YZ201354 and YZ201355) for their financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.talanta.2015.07.003.

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The fluorescence detection of glutathione by ∙OH radicals’ elimination with catalyst of MoS2/rGO under full spectrum...IntroductionExperimental sectionReagentInstrumentsPreparation of MoS2/rGOSample preparationFluorescence measurement

Results and discussionsCharacterization of MoS2/rGOOptimization of experimental conditionsAssay of GSHSelectivityApplication in practical samples

ConclusionAcknowledgmentSupplementary materialReference

the fluorescence detection of glutathione by ∙oh radicals...

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