A Label-Free Colorimetric Assay Based on Gold Nanoparticles for the Detection of H 2 O 2 and Glucose

: The signiﬁcance of sensing hydrogen peroxide (H 2 O 2 ) is due to its ubiquity, being a potential biomarker as well as an end-product of several oxidation reactions. Herein, based on gold nanoparticles (AuNPs) and coupled with single-stranded DNA (ssDNA) and ceria nanoparticles (CeO 2 ), we developed a novel colorimetric method to detect H 2 O 2 and glucose in NaCl solutions. In the presence of H 2 O 2 , ssDNA adsorbed on the surface of CeO 2 could be released and subsequently decorated AuNPs, resulting in a distinct color change of the aqueous solution from purple to red, which could be observed by the naked eye. Since H 2 O 2 can be produced in the process of glucose oxidation by glucose oxidase (GOx), this approach can also be employed to detect glucose. By employing this sensing system, the detection limits for H 2 O 2 and glucose are about 0.21 µ M and 3.01 µ M, respectively. Additionally, monitoring the content of glucose in blood serum samples was successfully achieved by the proposed strategy. This work opens a potential avenue for the quantitative detection of H 2 O 2 and glucose in clinical diagnostics.


Introduction
The detection of physiologically important species plays a pivotal role in clinical diagnosis and medical research. H 2 O 2 , as a crucial biomarker, is of practical significance in chemical, biological, and clinical fields [1][2][3]. In addition, H 2 O 2 is the final product in oxidation reactions and is catalyzed by glucose oxidase (GOx), cholesterol oxidase, and lactate oxidase, etc. [4,5]. It is essential to maintain the normal level of H 2 O 2 for various biological processes and the assessment of human health hazards. An excess of H 2 O 2 can induce various kinds of neurodegenerative diseases and biological damages in the human body; thus, accurate and rapid detections of H 2 O 2 are necessary to prevent further impacts on human health. Currently, although a number of existing methods such as PtPd-Fe 3 O 4 nanoparticles [6], boric-acid-functional [7], enzyme mimetics [8], hollow and porous Fe 3 C-NC nanoballoons (Fe 3 C-NC-NBs) [9], which are based on electrochemical approaches or fluorescence methods, have accomplished this goal, however, there are still challenges to develop neoteric techniques more adaptable to reliably detect H 2 O 2 , which do not rely upon advanced instruments and elaborately designed labels.
Moreover, it has a large surface-to-volume ratio and high extinction coefficient. Therefore, they have attracted extensive attention in colorimetric biosensors and diagnostics. In the past decades, a variety of AuNPs-based colorimetric assays for H 2 O 2 has been developed [23,24]. However, many of these methods lack sufficient sensitivity and simple operation. Thus, making great efforts in seeking a novel approach for H 2 O 2 monitoring is indispensable.
CeO 2 , as one of the most reactive rare earth materials, possesses many attractive properties that make it highly promising for special applications in photocatalysis, conversion catalysts, three-way catalysts (TWCs), solar cells, gates for metal-oxide semiconductor devices and phosphors, and colorimetric sensors [25][26][27][28][29][30][31]. Moreover, due to a great affinity of CeO 2 for H 2 O 2 , it may be considered for use in H 2 O 2 sensing systems with high sensitivity and selectivity [32].
As previously mentioned, H 2 O 2 is a product of glucose oxidation; thus, generally, the assays that monitor H 2 O 2 can be utilized for glucose detection. On the other hand, blood glucose concentration is the main basis for diabetes diagnosis, and it is also the main index of the patient's condition and degree of diabetes. For this reason, the detection of glucose is of great significance, resulting in the development of a variety of methods for the detection of glucose, such as mass spectrometry [33], electrochemistry [34][35][36], fluorescence [37,38], and colorimetric methods over the years. Among all of these methods, colorimetric methods have been investigated extensively due to their benefits such as simplicity, fast analysis speed, relatively low cost, sensitivity, and selectivity. Therefore, we developed a novel colorimetric method to detect H 2 O 2 and glucose based on AuNPs, ssDNA, and CeO 2 .
In the current study, we investigated interactions between H 2 O 2 and CeO 2 , combined with ssDNA and AuNPs in a high salt solution (Scheme 1). As illustrated in Scheme 1a, initially, ssDNA adsorbs on the surface of CeO 2 [32,39] when the ssDNA solution is mixed with a CeO 2 solution. Thereafter, in the addition of AuNPs solution and high concentration salt solutions, the aggregation of AuNPs occurs [40] due to there being no protection of ssDNA (presenting the macroscopical purple color. Upon the introduction of target H 2 O 2 , it induces ssDNA desorption from CeO 2 and binds to the surface of AuNPs, resulting in disaggregation and color change [24,41]. Hence ssDNA-decorated AuNPs can be finely dispersed (Scheme 1b), corresponding to the red color solution. When adding GOx, it is possible to monitor glucose even in blood serum samples. Thus, H 2 O 2 and glucose can be easily detected using the colorimetric method via the observation of remarkable color change, and their detection limits can reach 0.21 µM and 3.01 µM, respectively. Due to the use of biocompatible AuNPs and nonpolluting CeO 2 , the proposed strategy is relatively environmentally friendly. Moreover, it highlights a very promising strategy for the design of a new cost-effective and label-free family of H 2 O 2 and glucose colorimetric sensors.

Instruments
UV-vis absorption spectra of samples were measured by a Cary-5000 UV-vis spectrometer (Agilent) in a quartz cell (light path of 5 mm). The water signal, as the background signal, was taken away from the sample signal. Transmission electron microscopy (TEM) images were obtained by an HT-7700 instrument. The accelerating voltage was 120 kV. The particle size distribution diagram was obtained by Laser Particle Size and a Zeta Potential Analyzer. A FluoroMax-4 fluorescence spectrophotometer was used to determine the fluorescence spectra of samples. Under the condition of excitation at 490 nm with an Xe lamp, the fluorescence signals were recorded in the range of 505~640 nm. Slits for both excitation and emission measurements were 1.2 nm.

Preparation Procedures of AuNPs
AuNPs were fabricated by using trisodium citrate reductions in HAuCl 4 , following the procedures reported earlier [42][43][44]: (1) Ultrapure water (98 mL) and 50 mM HAuCl 4 (2 mL) were mixed in a three-neck flask and then heated to boil under stirring. (2) An amount of 38.8 mM trisodium citrate (10 mL) was transferred into the above flask rapidly. The mixture was heated in the condition of reflux with vigorous stirring for 15 min. During this process, the color of the solution changed from pale yellow to deep red gradually, which indicates the formation of AuNPs. (3) Centrifuge the solution for 15 min at a speed of 8000 r/min. The as-prepared AuNPs were redispersed with ultrapure water and stored at 4 • C for later use.
For the detection of glucose, the details were as followed: 100 µL of glucose solution with various concentrations (5, 0.5, 0.1, 0.05, 0.01, and 0.001 mM) was separately cultivated with GOx (100 µL, 0.05 mg/mL) at 37 • C for 40 min, which was subsequently introduced into the mixture of ssDNA and CeO 2 . Then, AuNPs (200 µL, 17 nM) and NaCl (200 µL, 0.2 M) solutions were successively added to the above solution. Finally, the resulting mixture was applied to spectroscopic measurements. In addition, 0.01 mM glucose was replaced by sucrose, galactose, and fructose with the same concentration to verify the specificity of this strategy.

Detection of Glucose in Diluted Serum Samples
The blood samples, collected from healthy adults (provided by a local hospital), were firstly centrifuged at 8000 r/min for 10 min to remove large aggregates and then diluted 500 times with HEPES buffer (pH = 7, 10 mM) without any other treatment before further experiments. HEPES (pKa at 20 • C: 7.45-7.65) is a zwitterionic, piperazinic buffer that is useful for a pH range of 6.8-8.2. In order to detect the content of glucose in diluted serum samples, the standard addition method was used [45]. Firstly, diluted serum (10 µL) and glucose solution (10 µL) with different concentrations were added into HEPES (80 µL) buffer. Then, the samples were utilized to detect glucose according to the same steps as mentioned above.

Characterizations of AuNPs and CeO 2
AuNPs fabricated by the trisodium citrate reaction method have an average diameter of~13 nm [42] (Figure 1a) and their solution displayed a distinct wine-red color with a distinctive absorption peak at approximately 523 nm ( Figure 1b). Therefore, the concentration of the AuNPs solution was evaluated to be about 17 nM according to the Lambert-Beer law with an extinction coefficient of 2.78 × 10 8 M −1 ·cm −1 for 13 nm AuNPs [46]. Some information on the size and morphology of CeO 2 can be obtained from Figure 2. TEM images indicate that CeO 2 was dispersed relatively well ( Figure 2a). The size distribution determined by DLS illustrates that the average diameter of CeO 2 is~20 nm (Figure 2b).

Mechanism of the Visual Assay for H2O2
In ordre to investigate the feasibility of the visual method for H2O2 sensing, A were used as a colorimetric substrate because of their low cost, nontoxicity, and e unaided visual detection. As observed from Figure 3(aI), the intensity of absorbanc trum of the (ssDNA + CeO2 + AuNPs + NaCl) system at about 523 nm is clearly re

Mechanism of the Visual Assay for H 2 O 2
In ordre to investigate the feasibility of the visual method for H 2 O 2 sensing, AuNPs were used as a colorimetric substrate because of their low cost, nontoxicity, and ease of unaided visual detection. As observed from Figure 3a(I), the intensity of absorbance spectrum of the (ssDNA + CeO 2 + AuNPs + NaCl) system at about 523 nm is clearly reduced, and a new peak emerges at around 625 nm. While adding H 2 O 2 , there only exists a strong absorption peak at 523 nm (Figure 3a(II)). Concomitantly, the color of the mixed solution changes from purple to red in the case of H 2 O 2 ( Figure 3a). Thus, in the presence of H 2 O 2 and high concentration salt solution, the adsorption of ssDNA on the surface of AuNPs will cause good dispersion of AuNPs along with red color solution. These data demonstrate the possibility of using (ssDNA + CeO 2 ) to directly detect H 2 O 2 using AuNPs in high concentration salt solutions.
To elucidate the mechanism of this sensing system for H 2 O 2 , FAM-labeled ssDNA was employed to determine the fluorescence spectrum. As shown in Figure 3b, it exhibits a distinct emission peak at~520 nm. Furthermore, the fluorescence of FAM-ssDNA was quenched obviously upon the introduction of CeO 2 , which may be due to adsorption of FAM-ssDNA. Significant fluorescence recovery was observed after adding H 2 O 2 . Since it was demonstrated by a previous study that H 2 O 2 binds to CeO 2 more strongly than FAM-ssDNA [32], we speculate that, in the presence of H 2 O 2 , FAM-ssDNA is desorbed from the surface of CeO 2 and released to the aqueous solution. Interestingly, when adding AuNPs into the mixed solution mentioned above, fluorescence quenching occurred immediately as a result of FAM-ssDNA adsorption on the surface of AuNPs [47].
was demonstrated by a previous study that H2O2 binds to CeO2 more strongly tha ssDNA [32], we speculate that, in the presence of H2O2, FAM-ssDNA is desorbed surface of CeO2 and released to the aqueous solution. Interestingly, when adding into the mixed solution mentioned above, fluorescence quenching occurred imm as a result of FAM-ssDNA adsorption on the surface of AuNPs [47].

Optimal Doses of ssDNA and CeO2 for the Assay
Previous studies have proven that AuNPs decorated with ssDNA can be fin persed when 0.2 M NaCl is introduced [20]. It is necessary to investigate the o concentration of ssDNA solution required to be adsorbed on the surface of AuNPs aqueous solution. In Figure 4a

Optimal Doses of ssDNA and CeO 2 for the Assay
Previous studies have proven that AuNPs decorated with ssDNA can be finely dispersed when 0.2 M NaCl is introduced [20]. It is necessary to investigate the optimum concentration of ssDNA solution required to be adsorbed on the surface of AuNPs in NaCl aqueous solution. In Figure 4a The effect of (ssDNA + CeO 2 ) on the existence state of AuNPs in 0.2 M NaCl solution was explored in the concentration range of 50~200 µg/mL CeO 2 . As shown in Figure 5a, when the concentration of CeO 2 increased to 200 µg/mL, the color of mixed solution transformed from red to purple. As observed in Figure 5b, compared to the absorption spectra of a relatively low concentration of CeO 2 , for 200 µg/mL CeO 2 , a new peak emerges at around 625 nm. These results suggest that AuNPs aggregation could occur in (ssDNA + CeO 2 + NaCl) system as CeO 2 was modified by ssDNA due to the coordination interaction between the ssDNA backbone and CeO 2 [48]. Based on this, 1.0 µM ssDNA and 0.2 M NaCl with a fixed concentration of CeO 2 (namely 200 µg/mL) can be utilized for the following experiments. of a relatively low concentration of CeO2, for 200 μg/mL CeO2, a new peak emerges at around 625 nm. These results suggest that AuNPs aggregation could occur in (ssDNA + CeO2 + NaCl) system as CeO2 was modified by ssDNA due to the coordination interaction between the ssDNA backbone and CeO2 [48]. Based on this, 1.0 μM ssDNA and 0.2 M NaCl with a fixed concentration of CeO2 (namely 200 μg/mL) can be utilized for the following experiments.

Colorimetric Detection of H2O2 and Glucose Using the Assay
According to the above results, we proposed a label-free and colorimetric method to monitor H2O2. In the presence of different concentrations of H2O2, the concentration-de-

Colorimetric Detection of H 2 O 2 and Glucose Using the Assay
According to the above results, we proposed a label-free and colorimetric method to monitor H 2 O 2 . In the presence of different concentrations of H 2 O 2 , the concentrationdependent color changes and the corresponding absorption spectra are exhibited in Figure 6a,b, respectively. The visual color of solutions undergoes a red-to-purple transformation, accompanied by an increased absorption at 523 nm (A 523 ) and a decreased absorption at 625 nm (A 625 ) with increasing concentrations of H 2 O 2 . Additionally, as observed from Figure 6c, the ratio of A 625 /A 523 shows a linear relationship (the linear relationship is Y = 0.3242 − 0.0538X, correlation coefficient of R 2 = 0.991) within the H 2 O 2 concentration range of 1 × 10 −8~0 .1 M. The limit of detection (LOD) of H 2 O 2 was calculated, using standard methods (LOD = 3 S/slope), to be 0.21 µM. S is the standard deviation of the blank sample. The slope was obtained from the standard curve ( Figure 6c). As illustrated in Table 1, the linear range of our strategy for H 2 O 2 detection is wider than that of the current methods. Moreover, the correlation coefficient of the linear equation for this method is higher than that of some reported approaches. Moreover, the sensitivity of our strategy is better than most of the current methods (Table 1). Although the LODs of nanostructured polysaccharide (cellulose nanowhiskers) CNW-AgNPs film [49], GQDs-AgNPs film [50], and Eu-MOF film [35] are lower than this work, there exist some aspects to improve. For example, the synthesis procedures of CNW and Eu-MOF are complex and the GQDs-AgNPs film contains AgNPs, which has higher biological toxicity. Therefore, the strategy proposed in this work has some obvious advantages to detect H 2 O 2 .    Glucose could be oxidized to produce H 2 O 2 and gluconic acid after the reaction of GOx and O 2 . Thus, this H 2 O 2 sensing assay was further used to monitor glucose. The results of glucose detection are presented in Figure 7a. As shown, visual color changes (red color to purple color) were observed with an increase in the content of glucose (0~5 mM). The corresponding UV-vis absorption spectra were determined and displayed in Figure 7b. As presented in Figure 7c, the A 625 /A 523 value decreases linearly (the linear relationship is Y = 0.6319 − 0.0984X, R 2 = 0.995) with the logarithm of glucose concentration (0.001~5 mM), based on which the LOD for glucose is estimated to be around 3.01 µM. In contrast, the correlation coefficient of the linear equation and the detection sensitivity of this assay is predominant over most other glucose-responsive detective methods (Table 2).    The specificity of this assay for glucose detection in the HEPES buffer was also evaluated under the same conditions by adding some individual possible interfering species (including sucrose, galactose, and fructose) and their mixture. As illustrated in Figure 8, only glucose can trigger obvious relative absorbance (I-I 0 )/I 0 , and I and I 0 refer to the absorbance ratio of A 625 /A 523 of the system with and without glucose, respectively. This demonstrates that the strategy proposed here shows considerable specificity for glucose detection in HEPES buffers. To test the feasibility of the proposed method, the analysis of glucose in huma rum samples was carried out by the standard addition method. The glucose concentr in blood samples measured by a commercial glucose meter is 4.3 ± 0.15 mM. The sele ity of the assay for glucose in diluted serum is shown in Figure 8. The mimetic sam contain the normal diluted serum and different concentrations of glucose. The recov range from 81.1% to 118% with the relative standard deviations (RSD) of 1.42%~1 (Table 3), indicating the potential applicability of this strategy for glucose detecti diluted human serum samples. RSD is calculated based on the results of three ex ments, according to the following equation.

RSD 100%
In Equation (1), S is the standard deviation of found concentrations in the thre periments, and denotes the average value of the found concentration. Table 3. Detection performance for glucose in healthy human blood serum. To test the feasibility of the proposed method, the analysis of glucose in human serum samples was carried out by the standard addition method. The glucose concentration in blood samples measured by a commercial glucose meter is 4.3 ± 0.15 mM. The selectivity of the assay for glucose in diluted serum is shown in Figure 8. The mimetic samples contain the normal diluted serum and different concentrations of glucose. The recoveries range from 81.1% to 118% with the relative standard deviations (RSD) of 1.42%~1.98% (Table 3), indicating the potential applicability of this strategy for glucose detection in diluted human serum samples. RSD is calculated based on the results of three experiments, according to the following equation.
In Equation (1), S is the standard deviation of found concentrations in the three experiments, and X denotes the average value of the found concentration.

Conclusions
To summarize, a label-free and visual colorimetric assay was developed for the sensitive detection of H 2 O 2 based on the interaction between it and CeO 2 . Without H 2 O 2 , ssDNA decorates the surface of CeO 2 and, almost simultaneously, AuNPs aggregate in the high salt solution. While introducing H 2 O 2 , it can combine with CeO 2 more firmly. In the presence of H 2 O 2 , ssDNA bound to CeO 2 is released to bind aggregated AuNPs, causing them to disaggregate, resulting in a purple-to-red color change that can be easily observed by the naked eye. In addition, since H 2 O 2 could be generated from the oxidization of glucose by GOx, this strategy could be also utilized to detect glucose even in blood serum samples. Compared with most other methods, this method has higher sensitivity, wider detection range, and lower LOD. This work provides a novel method to monitor H 2 O 2 and glucose, which has a promising application in the early diagnosis of some related diseases.