Synthesis of glycine-functionalized graphene quantum dots as highly sensitive and selective fluorescent sensor of ascorbic acid in human serum

Accepted Manuscript Title: Synthesis of glycine-functionalized graphene quantum dots as highly sensitive and selective fluorescent sensor of ascorbic acid in human serum Author: Rui Liu Ran Yang Chaojie Qu Haichen Mao Yue Hu Jianjun Li Lingbo Qu PII: DOI: Reference:

S0925-4005(16)31723-3 http://dx.doi.org/doi:10.1016/j.snb.2016.10.096 SNB 21154

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

20-6-2016 4-10-2016 20-10-2016

Please cite this article as: Rui Liu, Ran Yang, Chaojie Qu, Haichen Mao, Yue Hu, Jianjun Li, Lingbo Qu, Synthesis of glycine-functionalized graphene quantum dots as highly sensitive and selective fluorescent sensor of ascorbic acid in human serum, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.10.096 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of glycine-functionalized graphene quantum dots as highly sensitive and selective fluorescent sensor of ascorbic acid in human serum Rui Liu1, Ran Yang1, Chaojie Qu1, Haichen Mao1, Yue Hu1, Jianjun Li1, Lingbo Qu1 1The

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou

450001, PR China 2School

of Environmental Engineering and chemistry, Luoyang Institute of Science and

Technology, Luoyang 471023, PR China

∗

Corresponding author at: The College of Chemistry and Molecular Engineering, Zhengzhou University, Kexue Road, Zhengzhou 450001, PR China. E-mail: [email protected]; [email protected] .cn (L. Qu)1



Ran Yang, E-mail:[email protected] Qu,E-mail:[email protected]

Lingbo

Highlights: (1) A novel and highly photoluminescence glycine-functionalized graphene quantum dots were synthesized and characterized. (2) The obtained glycine-functionalized graphene quantum dots were noted as a fluorescence probe for the determination of ascorbic acid. (3) Relative to the reported methods, the proposed method in our work are more sensitive and selective for the determination of ascorbic acid. (4) The proposed method is much simpler and faster, a whole detection just needs 8 minutes.

ABSTRACT In this work, highly photoluminescent glycine (GLY) functionalized graphene quantum dots (GQDs) (GLY-GQDs) were synthesized by a simple and green pyrolysis method employing ethylene glycol as carbon source, GLY as functional molecule. The as-synthesized GLY-GQDs exhibited excellent water solubility with a fluorescence quantum yield of 21.7%. The fluorescence of GLY-GQDs was intensively quenched by Ce4+ via forming non luminescent complexes of GLY-GQDs-Ce4+. When ascorbic acid (AA) was in presence, Ce4+ was reduced to Ce3+ and the fluorescence of GLY-GQDs regained. In the light of this theory, a simple AA sensor was fabricated without complicated, costly and time-consuming operations. Under the optimal conditions, the fluorescence recovery ratio and the concentration of AA has a linear relationship in the range of 0.03-17.0 µM and with a detection limit of 25 nM which

was one order higher sensitive than the reported methods. Furthermore, this established sensor system also shows a high selectivity toward AA over a wide range of common biological molecules such as uric acid, dopamine and glutathione and so on. The proposed method was successfully applied for the AA detection in serum samples. All these suggested the potential of this GLY-GQDs based sensor in the clinical analysis. Keywords: Glycine functional graphene quantum dots; Cerium (Ⅳ); Ascorbic acid; Serum samples; fluorescence sensor.

1. Introduction Ascorbic acid (AA, vitamin C), an important endogenous antioxidant, exists widely in human physiological environment [1]. It acts as an important intracellular reduction oxidation buffer to scavenge physiological free radicals and peroxides and to prevent damage [2]. When the inner environment is under oxidative stress, AA would be oxidized to its oxidative form. Amount of AA in physiological inner is associated with numerous clinical diseases such as scurvy, Parkinson's disease, urinary stone, diarrhea, stomach, as well as numerous types of cancers [3, 4]. Therefore, monitoring the concentration of AA in biological samples is of great significance for medical diagnosis. Up to now, there are a number of methods have been developed for the detection of AA, including chromatography (HPLC) [5, 6], capillary electrophoresis [7, 8],

electrochemistry [9, 10] and colorimetry [11]. Although these methods have exhibited promising results for AA detection, there are still some limitations such as the time-consuming process, the utilization of sophisticated and specialized equipment, and requirement for skilled technicians. Comparably, fluorescence methods for determining AA have attracted more attention because of their simplicity, convenience and high sensitivity. With the help of different fluorescence probes, many fluorescence sensors for the determination of AA have been reported [12-24]. According to the detection mode, the reported methods can be divided into two categories, one kind is “off” sensor based on the directed quenching effect of the AA to the probe [12-15]; the other kind is switch-on sensor based on the fluorescence recovery of AA to the quenched probe [16-21]. The “off” sensor is easily caused false-positive results due to other quencher or environmental stimulus. Relative to the “

off

”

sensors, the switch-on sensor was more selective and sensitive. Based on the

special reaction of AA and all kinds of quenchers, such as Fe(III), Cu(II), Cr(VI) , MnO4- and MnO2, many “switch-on” sensors had been reported. These switch-on fluorescence sensors made great progress in analysis AA in biological samples. However, in these reports, there are some problems need to be improved further such as harsh synthesis conditions of probes, complicated probe fabrication process, application of non-environmentally friendly probe, low sensitivity and selectivity and so on. Therefore, Exploring novel fluorescent sensor with ideal bio- and environment safety nano-material, simply operability, high sensitivity and selectivity and low-cost property has become an urgent challenge.

Grapheme quantum dots (GQDs), as a new-style quantum dot system, offers strong potential for promising candidate to replace traditional semiconductor quantum dots [22, 23]. They have drawn increasing attention in sensor application due to their low toxicity, excellent photostability, superior biocompatibility, highly tunable photoluminescence (PL) property and chemical inertness [24]. Liu et al [21] developed a sensitive method for AA detection based on the GQDs-Cu (II) system, however, the selectivity of the above method needs further improvement because UA, DA with same concentration as AA have more than 15% interference. With respect to GQDs, functionalized GQDs have higher quantum yield and better analysis performance [25-27]. Glycine is the simplest amino acid with one carboxyl group and amino group. Such structure with rich functional groups makes it very easy to make non-covalent interaction other molecule which suggested great potential of GLY as a good functional molecule to improve the analytical performance of GQDs. Ce(IV) is a special cation with strong oxidizing and coordination property and widely used as the quencher of the fluorescence probe [25]. Relative to the reported quenchers in the off-on sensors of AA, such as Fe (III), Cr(VI), Cu2+, MnO4- and MnO2, Ce(IV) has more stronger oxidation property. In this case, if Ce(IV) was used as the quencher to fabricate switch-on sensor of AA, the analytical performance of the sensor would be great improved because there is greater reaction capacity between AA and Ce(IV). Therefore, in this work, GLY functionalized GQDs (GLY-GQDs) was synthesized and used as the fluorescence probe of AA, based on the quenching effect of Ce(IV) to GLY-GQDs and the special redox reaction between AA and Ce(IV), a very sensitive

and selective fluorescence sensor of AA was developed (Scheme 1). Compared to the reported publications, not only the whole synthesis procedure of probe got more simper and shorter and it just need few minutes, but also the sensing performance of probe was more superior. The proposed method has an order of magnitude higher sensitivity and much better selectivity for the interference of DA, UA and GSH. This is the first time to application GLY-GQDs in fluorescence sensing. Our demonstration may open up new avenues of GQDs for future biomedical sensing and other optical applications.

2. Materials and methods 2.1 Reagents and materials Ascorbic acid, Uric acid was purchased from Alfa Aesar. Glutamate, methionine, L-Glutathione (reduced) were purchased from Solarbio. Ethylene glycol, Glycine Histidine, arginine, glycine, Leucine, L-tryptophan, Lysine, glucose were purchased from Aladdin. All other reagents are of analytical reagent grade. Standard stock (0.1M) solution of Ce4+ ions was prepared with Ce(SO4)2 in 0.1 M H2SO4. Different concentrations of Ce4+ ions were obtained by diluting standard stock solutions. 0.1 M acetic-acetate buffer was used throughout the experiment.

2.2 Instrumentations and measurements TEM images were acquired on transmission electron mircroscopy (FEI-Tecnai G2, USA )at an accelerating voltage of 200 kV. X-ray photoelectron spect

roscopy (XPS) was performed on a Thermo-VG Escalab 250. All fluorescence spectra were surveyed on an F97 fluorescence spectrophotometer (ShangHai, China). UV-vis measurements were carried out on a TU-1810 spectrophotometer (Beijing, China) an d the spectra were collected from 220 nm to 700 nm. The time-resolved fluorescence decay and the fluorescence quantum yield were performed on an Edinburgh FLS980 s pectrofluorometer (Edinburgh, UK) with a 360 nm LED as the excitation source.

2.3 Synthesis of GLY-GQDs GLY-GQDs were prepared by a pyrolysis method with one-step. 0.3 g glycine were added into 10 mL ethylene glycol in a 20 mL round-bottom flask and heated from room temperature to 170 ℃ in a heating mantle with stirring. When the color of the liquid was changed from colorless to yellow, and then brownish red, the heating was finished and the flask was taken out and cooled down to room temperature naturally. The cooled solution was then centrifuged at 10,000 rpm for 10 min to remove some little amount of insoluble matter; the obtained supernatants are GLY-GQDs. The GLY-GQDs stock solution was prepared by diluted with same volume redistilled water.

2.4 The sensing procedure 60 µL 0.1M acetic-acetate buffer (pH=5.5), 30 µL GLY-GQDs stock solution and 210 µL 0.01M Ce4+ were spiked into fluorescence cuvette and diluted with distilled water to get a final volume of 3.00 mL, After reaction for 4 min, different volume AA stock solution were added into the cuvette and reaction 4 min. The cumulative volume of AA was not beyond 50 µL. The fluorescence intensity was

measured at λem/λex = 448/360 nm.

3 Results and discussions 3.1 Synthesis and characteration of GLY-GQDs The melting point of ethylene glycol is -12.9 ℃, it appears as a liquid state at room temperature. Relative to the synthesis procedure with citric acid as carbon source(its melting point:153℃), the synthesis process is greatly shorted because it does not require the procedure for absorption and melting, so, in our work, the whole reaction process just cost several minutes. The TEM images of GLY-GQDs showed that the diameters of GLY-GQDs were distributed in the range of 3-5 nm (Fig. 1a). By the high-resolution TEM (Fig. 1a), most particles are observed to have a discernible lattice structure. Various lattice planes can be clearly identified with spacings of 0.24 nm, which correspond to the (103) diffraction plane of graphene ((100) facet). The spectroscopy (XPS) spectra (Fig. 1b) of the GLY-GQDs exhibit three peaks at 285.48 eV, 399.17 eV and 530.68 eV which are attributed to C 1s, N 1s, and O 1s, respectively. The high-resolution XPS spectra of C 1s (Figure 1c) can be well deconvoluted into four surface components, corresponding to sp2 (C=C/C−C) at binding energy of 284.0 eV, C-N at 285.4 eV, C-O at 287.1 eV, and C-O at 287.8 eV. The atomic percentage of C, N and O were found to be 54.3％, 21.15％, 24.55％, respectively. The high-resolution N 1s spectrum of GLY-GQDs (Fig. 1d) shows that there are two peaks at 399.0 eV and 399.6 eV, corresponding to C-N and N-H [26]. From the above mentioned, we can concluded that the GLY functionalized GQDs have been successfully synthesized.

3.2 Optical properties of GLY-GQDs Fig.2a shows the UV-vis absorption and photoluminescence spectra of GLY-GQDs under different excitation wavelengths. As shown, GLY-GQDs have two absorption bands: one located at 201 nm likely originates from the formation of multiple polyaromatic chromophores, and the other one centered at 328 nm is ascribed to the n-π* transition of GLY-GQDs. The results of photoluminescent spectra of GLY-GQDs indicate that the emission wavelength of GLY-GQDs is nearly excitation-independent (Fig.2b). With increasing excitation wavelengths, the emission peak position shifts to longer wavelengths and the intensity decreases rapidly. The maximum emission intensity from GQDs was achieved at 448 nm when they excited at 360 nm. The absolute quantum yield at 360 nm excitation was calculated to be ~21.72％ by FLS 9 8 0. Under visible light for two months, the RSD of the fluorescence intensity at 435 nm of GLY-GQDs is not more than 5% suggesting the high stability of the as-prepared GLY-GQDs.

3.3 The mechanism of “off-on” mode of GLY-GQDs-Ce4+-AA As shown in Fig. 3a, the fluorescence of GLY-GQDs gradually decreased with the adding of Ce4+ when the concentration of Ce4+ was up to 0.7 mM, up to 93.0 % quenching of the fluorescence emission was observed. Such a quenching may be ascribed to the coordination of Ce4+ and the functional groups of GLY-GQDs such as the carboxyl group, hydroxyl group and amino group. When AA was in the presence,

the fluorescence of GLY-GQDs-Ce4+ gradually being restored with the increase of AA and the emission peak position also showed a blue shift from 448 nm to 435 nm (Fig. 3b). The reason may be that AA is a strong reducing agent, and it could reduce Ce4+ to Ce3+ Relative to Ce4+, the property of coordination of Ce3+ with the functional groups decreased greatly, in this case, the GLY-GQDs could be released and the quenched fluorescence of the GLY-GQDs by Ce4+ can be recovered. To confirm the mechanism of the above process, the quenching of Ce4+ for the GLY-GQDs was first investigated. As we all know, the reason for the fluorescence quenching is dynamic quenching effect (DQE) or static quenching effect (SQE) or both simultaneously. In DQE, the excited-state fluorophore is nonradiatively deactivated upon collision with the quencher. In SQE, the fluorophore forms a nonfluorescent complex or with the quencher. DQE could be theoretically described by Stern-Volumer equation[27]： F0/F = 1 + Ksv [Q] = Kq τ0 Where, [Q] is the concentration of quencher ie, Ce4+ in this study; F0 and F are the steady-state fluorescence intensities in the absence and presence of the quencher, respectively, Ksv is quenching constant; τ0 is fluorescence lifetime; Kq is the molecular quenching rate constant. For DQE, the maximum Kq is 1.0×1010 M-1 S-1 [28]. In our study, the fluorescence intensity ratio of Ce4+ in the absence and presence of Ce4+ was linear with the concentration of Ce4+ could be described as: F0/F = 1.01 + 0.0040c (c: µM, R2=0.9978) suggesting the Ksv = 0.0040 µM

-1

. The time-resolved

fluorescence decay of GLY-GQDs, GLY-GQDs-Ce4+ were show in Fig. 4a, their life

time were 6.32, 5.50 ns, respectively. According to the lifetime of GLY-GQDs, the Kq of GLY-GQDs-Ce4+ was calculated as 6.2×1011 M-1 S-1, which is much larger than maximum Kq of DQE suggesting the SQE for the quenching effect of Ce4+ on GLY-GQDs. The UV-Vis absorption spectrum of GLY-GQDs and GLY-GQDs+Ce4+ (Fig. 4b) showed that when Ce4+ was added into GLY-GQDs, the absorption peak of GLY-GQDs at 328 nm disappeared which suggested the forming of the new chemical bond. Combined with the XPS results that the GLY-GQDS has the functional groups such as carboxyl, hydroxyl and amino we could concluded that Ce4+ and GLY-GQDs formed the non-fluorescent complex and caused a SQE. The UV-Vis absorption of AA, Ce3+, GLY-GQDs + Ce4+ and GLY-GQDs + Ce4+ + AA were shown as Fig. 4b. It could be seen that when AA was added into the solution of GLY-GQDs+Ce4+, the disappeared absorption peak of GLY-GQDs at 328 nm restored and the absorption peak of Ce3+ appeared which confirmed the redox reaction between AA and Ce4+ and Ce3+ could not form effectively non-fluorescent complex with the GLY-GQDs.

3.5 Optimum the conditions To get the better analysis behaviors, the pH and consumption of buffers, reaction time of GLY-GQDs-Ce4+ and GLY-GQDs + Ce4+ + AA were optimized. According to the recovery rate of fluorescence (F-F0/F0), the above determination conditions were optimized. It could be seen that whether GLY-GQDs-Ce4+ or GLY-GQDs-Ce4+-AA could rapidly reaction completely in 4 min and kept stable during the following 20

min (Fig. 5a) implying a promising application in a fast sensing of AA without strict time control. The buffer consumption and pH value affected not only the fluorescent intensity of the original GLY-GQDs-Ce4+ solution, but also the subsequent fluorescence recovery by AA. The recovery rate of fluorescence of GLY-GQDs-Ce4+ in 50 uL 0.1M ABS (Fig. 5b) with pH 5.5 (Fig. 5c) is higher than those of other pH values, so 50 uL pH 5.5 was chosen for subsequent study.

3.6 Sensitivity of the sensing system To ensure the presented system can be used for sensitive quantification of AA, the fluorescence responses of GLY-GQDs + Ce4+ induced by AA at different concentrations were evaluated (Fig. 6a). Under the optimum conditions, the fluorescence enhancement factor (F-F0)/F (Fig. 6b) (F0: the intensity of GLY-GQDs + Ce4+; F: the intensity of GLY-GQDs + Ce4++AA) was lineared with the concentration of AA in the range of 0.03 to 3.3µM and 3.3µM to 16.7 µM, the regression equation are y=0.29915 C (µM) + 0.08558 and y=0.03724 C (µM) + 0.98027, with correlation coefficients (R2) of 0.99452, (R2) of 0.98092, respectively. Where Y is the fluorescence enhancement factor.

The detection of limit was estimated to be 25 nM

(3σ/S, in which σ is the standard deviation for the blank solution, n = 10, and S is the slope of the slope of the calibration curve) [29-31]. The comparison of the analytical performance of AA determination at the developed fluorescence probe with other methods reported previously is summarized in Table 1. It can be seen that the present GLY-GQDs nanoprobe exhibited a wider linear range and a lower detection limit,

especially, the sensitivity in our work has an order magnitude higher than the other reported carbon dots and graphene quantum dots suggesting the better performance of GLY-GQDs as a fluorescence probe. This could be ascribed to the greater reaction capacity between AA and Ce4+. All these proved our design concept and also may bring new concept to design novel sensor with more superior performance.

3.7 Selectivity of the sensing system In order to evaluate the detection specificity of the present detection system, the degree of other biologically relevant reactive species, such as metal ions and some biomolecular and reducing agents was added into the detection system of GLY-GQDsGLY-GQDs + Ce4++AA, the concentration of AA is 10 µM, the fluorescence intensity of GLY-GQDs + Ce4++AA with and without the interference were marked as F and F0, respectively. The ratio of (F-F0)/F was used to evaluate the effect of the interference on the detection system. It could be seen that (Fig. 7a and Fig. 7b), except Fe3+, common metal ions with a concentration of 100 bold of AA, most of amino acids with a concentration of 20-bold AA had no effect on the system. To eliminate the interference of Fe3+, as shown in Fig. 7c, the chelating agent of adenosine triphosphate (ATP) was added into GLY-GQDs, it could be found that 0.6 mM ATP had no influence on the fluorescence of GLY-GQDs and GLY-GQDs-Ce4+, but it could eliminated the interference of 5 mM Fe3+, so, in the experiment, we can adding appropriate ATP to eliminate the interference Fe3+. Because the mechanism of fluorescence back of AA to GLY-GQDs + Ce4+ was

originated from the oxidation-reduction behaviors, so, the effect of some other co-exsiting reducer such as GSH, DA, UA also were investigated. One times UA and 10 times GSH had no interference on the GLY-GQDs- GLY-GQDs + Ce4++ AA. Through the ratio of (F-F0)/F after adding DA was beyond 10%, their concentration in the biobody is just about one percent of AA, so, in the real analysis, their interference also may be negligible. As a result, the proposed method showed an excellent selectivity and could be used for the biological samples analysis.

3.8 Real sample assay Human serum samples were taken from a volunteer of our laboratory. 4.0 mL of acetonitrile was added to 1.0 mL of human plasma and centrifuged at 10,000 rpm for 10 min, the supernatant was collected and dried with nitrogen. The residue was dissolved with 1.0 mL ABS buffer and then detected as the 2.4 described. The concentration of AA was detected as 2.56 µM which is accorded with the AA concentration in biobody [15]. Different concentration AA with a high, medium and low concentration level were added into pretreated plasmas, the recovery rate were range from 99.90 ％ to 100.02 ％ (Table 2), suggesting the applicability of the proposed method to real analysis of AA in biological samples.

4. Conclusions In this work, GLY-GQDs were synthesized by a very simple and fast pyrolysis method. The obtained GLY-GQDs were water-soluble, high quantum yield and stable against light illumination and long time placement. The fluorescence of GLY-GQDs

could be quenched by Ce4+ and then recovered by AA due to the coordination and redox among GLY-GQDs, Ce4+ and AA. Based on this, a novel fluorescence sensor of AA with “off-on” detection mode was developed. Relative to the reported methods, the proposed method not only showed higher sensitivity, selectivity, but also was much simpler and faster, a whole detection just need 8 minutes, all these suggested the promote application of GLY-GQDs in the biological analysis.

Acknowledgments

This work was supported by National Natural Science Foundation of China U140424 and J1210060 and Science and technology project of Henan province (152102410006)

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Biographies Rui Liu is currently pursuing a Ph.D. degree in analytical chemistry under the supervision of Professor Ling-bo Qu and Ran Yang associate professor. Her research area is nanomaterials and their applications in sensor. Ran Yang is a associate professor of College of Chemistry and Molecular Engineering, Zhengzhou University. Her research interests are focused on biochemistry and bioanalysis. Chaojie Qu is currently a MS candidate in Zhengzhou University. Her research area is nanomaterials and their applications in sensor. Haichen Mao is currently a MS candidate in Zhengzhou University. His research area is nanomaterials and their applications in bioanalysis . Yue Hu is a undergraduate in Zhengzhou University, her major is analytical chemistry. Jianjun Li is a professor of College of Chemistry and Molecular Engineering, Zhengzhou University. His research interests are focused on bisensors and electroanalytical chemistry. Ling-bo Qu is a professor of School of Chemistry & Chemical Engineering, Henan University of Technology. He received his Ph.D degree in pharmaceutical chemistry from China Pharmaceutical University. His research interests are focused on computational chemistry, pharmaceutical analysis and bioanalytical chemistry.

Figure Captions:

Fig. 1. Synthesis and characteration of GLY-GQDs. (a) TEM and HRTEM images of GLY-GQDs. (b) XPS spectra of GLY-GQDs. (c) High-resolution C 1s peaks. (d) High-resolution N 1s peaks.

Fig. 2. The optical properties of GLY-GQDs. (a) UV and fluorescence spectra of GLY-GQDs. (b) Fluorescence emission spectra for the GLY-GQDs at different excitation wavelengths.

Fig. 3. (a) Fluorescence emission spectra of GLY-GQDs in the presence of several of Ce4+ (0, 0.05, 0.2, 0.3, 0.5, 0.7 mM); (b) Fluorescence emission spectra of GLY-GQDs at different conditions: a) GLY-GQDs; b) GLY-GQDs + AA (50µM); c) GLY-GQDs + Ce4+ (0.7 mM); d) GLY-GQDs + Ce4+ (0.7 mM) + AA (50 µM).

Fig. 4. (a) Time-resolved fluorescence decay of GLY-GQDs in the absence and presence of Ce4+(0.7 mM) and AA (100 µM). (b) The UV-Vis absorption spectrum.

Fig. 5. (a) a) Fluorescence quenching of GLY-GQDs by Ce4+ as a function of time ,and b) fluorescence restoration of GLY-GQDs containing Ce4+ (0.7 mM) by AA (2 µM) as a function of time. (b) Optimized the volume of ABS. (C) Optimized the value of ABS.

Fig. 6. (a) Fluorescence recovery spectra of GLY-GQDs containing Ce4+ (0.7 mM) by addition of AA with concentration ranging from 0 to 16.7 µM and (b) linear relative fluorescence intensity and AA concentration.

Fig. 7. (a) Selectivity of sensing system towards AA and other biological relevant reactive species. Experimental conditions: 10 µM AA, dopamine (DA), urine acid (UA); 100 µM glutathione (Reduced) (GSH); (b) 200 µM L-phenylalanine, histidine, arginine, glycine, leucine, L-cysteine, glucose; 1000 µM Ba(NO3)2, MgCl2, KCl, CuCl2, CaCl2, AgNO3, Al2(SO4)3, CdCl2, ZnSO4, FeCl3. (c) Masking Fe (Ⅲ) ions. a) GLY-GQDs; b) GLY-GQDs + ATP (0.6 mM); c) GLY-GQDs + Fe3+ (5 mM); d) GLY-GQDs + Fe3+ (5 mM)+ ATP (0.6 mM); e) GLY-GQDs + Ce4+ (0.7 mM)+ ATP (0.6 mM).

Scheme 1 “off –on” fluorescence system of GLY-GQDs-Ce4+-AA.

v

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Scheme 1

Table 1. Comparison of different probes for AA determination. Probe

Detection mode

Linear range

Detection

Sample

reference

BSA-Au NCs

Off

1.5-10μM

0.2μM

Human blood plasma

[12]

Ag NPs

Off

4.1-100 μM

0.1μM

SiPc–TEMPO

Off

~2 mM

N-Carbon NPs

off

0.2-150µM

(g-C3N4)- Cr(Ⅵ)

off -on

CdTe QDs-KMnO4

vegetables and vitamin C tablets.

[13]

Cancer cells

[14]

50 nM

Human biological fluids

[15]

0.5-200µM

150 nM

biological fluids

[16]

off -on

0.3-10µM

74 nM

biological fluids

[22]

7-hydroxycoumarin -MnO2

Off-on

0.5-40μM

0.09μM

cerebral samples

[24]

PLNP-CoOOH

Off-on

1-100μM

0.59μM

in living cells

[17]

DNA-fluorescein/da bcyl-AuNPs

Off-on

Not given

2.5 μM

vitamin C tablet, urine, orange juice

[18]

Off-on

0.18-90μM

42nM

in food samples

[19]

CQDs-Fe(III)

Off-on

0.1-50µM

9.1 nM

Rat brain microdialysates

[23]

CDs-Cr(VI)

Off-on

30-100µM

1220 nM

NO-given

[20]

GQDs-Cu (II)

off -on

0.3-10µM

94 nM

vitamin C tablet

[21]

GLY-GQDs-Ce (Ⅳ)

Off-on

0.03-16.7 μM

25 nM

Human serum

This work

CQDs-MnO2

Table 2. Determination of AA spiked in human serum samples (n=3) Human serum

Concentration

Concentration

Recovery

RSD

samples

added (µM)

found (µM)

％

％

0

0.521

/

2.943

2

2.519

99.90

3.587

5

5.520

99.98

2.476

10

10.523

100.02

1.231

Human serum