Boric Acid-Based Dual Modulation Photoluminescent Glucose Sensor Using Thioglycolic Acid-Capped CdTe Quantum Dots
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Most luminescent glucose sensors based on the interaction of glucose with organic boric acids or borates. Herein, a new luminescent glucose sensor is designed using thioglycolic acid-capped CdTe quantum dots in the presence of cheap inorganic boric acid. Both peak position and intensities change upon the addition of glucose because of the interaction of boric acid with glucose and thioglycolic acid-capped CdTe quantum dots, which enables glucose detection by either color change or intensity change. The luminescent intensities change linearly with glucose concentrations in the ranges from 0.03 to 1 mM and 1–25 mM with a detection limit of 10 µM (S/N = 3). Moreover, glucose concentrations can be conveniently detected by color change in the range from 1 mM–25 mM. It displays a highly selective response to glucose over other interfering but biologically important saccharides, amino acids, and common ions.
KeywordsCdTe quantum dots Glucose sensor Thioglycolic acid
Organic and inorganic boronic acids behave as lewis acids and can form complexes with other moieties such as amino acids, carbohydrates, nucleotides, and hydroxyl acids through electron donor–acceptor interactions. They can produce tetrahedral anionic complexes through covalent and reversible condensation reactions with diols and related divalent ligands . The borates show favorable affinity and selectivity with respect to different saccharides, such as glucose [2, 3].
The sensing of glucose is very important, and has drawn much interest as the physiological breakdown of glucose is related to many diseases including diabetes, cancers, and physiological aging and neurodegenerative diseases [4, 5, 6]. Therefore, a large number of methods have been developed to detect glucose using organic probes functionalized with borates/boronic acids , such as phenylboronic acid , anthrylboronic acid , ferrocenylboronic acids , cyanine dye possessing boronic acid , boronic acid gel , boronic acid functionalized benzyl viologen derivative, and/or boronate-functionalized polymeric monoliths over past few decades [2, 13]. These probes have been reported to provide sensitive and selective alternative platforms for the recognition of glucose through reversible covalent interaction between diols and borates centre [2, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. In comparison with organic luminescent probes, quantum dots (QDs) have the advantages of higher photostability, continuous absorption spectra, and size-dependant emission properties [27, 28]. It has received increasing attention for glucose detection by coupling with borate functionalization. The QD-based luminescent glucose sensors outlined are generally based on organic borate species [2, 3, 29, 30, 31]. However, QD-based luminescent glucose sensors using inorganic borates have rarely been used .
In the present work, we design a new photoluminescent glucose sensor using inexpensive inorganic boric acid and water-soluble thioglycolic acid-capped CdTe QDs (TGA@CdTe QDs) in aqueous solution at physiological pH. The interaction between TGA@CdTe QDs and boric acid results in the formation of B(OH)3–TGA@CdTe QDs, leading to red shift in peak position and decrease in the PL intensity of QDs. The addition of glucose further modulates both peak intensity and peak position of B(OH)3–TGA@CdTe QDs towards longer wavelength because of the interaction of glucose with boric acid. The proposed sensor provides dual PL modulation in terms of emission intensity and wavelength shift (color change). The effect of other interfering species including saccharides, amino acids, and ions on the detection of glucose has also been investigated. The present switchable sensor offers a simple method to sense glucose visually under UV lamp without the need of any covalent modification of boronic acids and borates with organic luminescent probes or QDs.
2 Experimental Section
Thioglycolic acid (90%), CdCl2·5H2O (99%), boric acid, rongalite, and Rhodamine 6G (with a photoluminescence quantum yield of 95% in anhydrous ethanol) were purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Tellurium (Te) powder was obtained from Aladdin Chemistry Co., Ltd, Shanghai, China. All other chemicals were of analytical grade and used as received. Double deionized water was used throughout the experiment.
2.2 Instrumentation and Characterization
The FTIR spectra of QDs were recorded with a Bruker Vertex 70 Fourier transform-infrared spectrometer (Beijing Scientific Technology Co., Ltd). FEI Tecnai G2 F20 TEM microscope was operated at 200 kV accelerating voltage. 3 µL of aqueous solution was deposited onto a copper TEM grid and dried at room temperature, and then was utilized for TEM study. DLS measurements were performed in aqueous solutions using Zetasizer Nano ZS (Malvern Instruments Ltd, Worcestershire, UK) at 28 °C. PL spectra were recorded with 400 nm excitation using Perkin–Elmer LS 55 luminescence spectrometer. Excitation and emission of slits were 5.0 and 10.0 nm, respectively. UV–visible spectra were obtained using a CARY 500UV/Vis–near-IR spectrophotometer (Varian). Samples for absorption and emission measurements were contained in 1 cm × 1 cm quartz cuvettes (3.5 mL volume, from Hellma).
2.3 Preparation of TGA@CdTe QDs
Te2− precursor solution was synthesized as follows: Mix 0.0013 g Te powder with 3 mL deionized water in 50 mL round-bottom flask equipped with a stirrer, an N2 gas inlet, and a condenser. 0.023 g rongalite (3.53 M) was then added into the flask. The pH of the solution was maintained at 11.6 by adding NaOH. The mixture was stirred at 100 °C for 40 min. To prepare TGA@CdTe QDs’ solution, the freshly prepared N2-saturated CdCl2·2.5H2O/TGA solution (pH = 11.6) was injected rapidly into refluxing flask containing Te2− precursor solution under nitrogen atmosphere. TGA@CdTe QDs’ photoluminescence quantum yield was obtained after 30 min reflux of the precursor solution at 100 °C. When the temperature cooled down to room temperature (~20 °C), TGA@CdTe QDs were precipitated with 2-propanol and collected via centrifugation (Anke TGL16C high-speed desktop centrifuge) for 5 min at 10,000 rpm and 20 °C. As-synthesized TGA@CdTe QDs were redispersed in 5 mL deionized water and used in subsequent steps.
2.4 Preparation of B(OH)3–TGA@CdTe QDs’ sensor
B(OH)3–TGA@CdTe QDs was prepared by dissolving 0.064 g of B(OH)3 in 4 mL water to get final concentration of 1.25 µM at pH 7.4, and then adding as-prepared TGA@CdTe QDs (50 µM). The mixture was kept for 3 h at room temperature (~25 °C). The resultant products were centrifuged and washed with water and ethanol. The product was redispersed in waster for further analysis.
2.5 Calculation of PL Intensity Response
2.6 Glucose Determination
B(OH)3–TGA@CdTe QDs (1.25 mM) was used for measurement of glucose concentrations. Various concentrations of glucose solution were added and absorbance and PL quenching were measured immediately. The blank determination (P 0) was carried out without adding glucose. For interference study, aqueous buffer solutions (pH = 7.4) mixed with a variety of saccharides, amino acids, and ions were prepared. The interference studies were carried out by adding appropriate concentration of each interfering species in the presence of glucose. The emission spectra were recorded from 500 to 800 nm upon excitation at 400 nm for all measurements. All experiments were performed in triplicate.
3 Results and Discussion
3.1 Synthesis and Characterization of B(OH)3–TGA@CdTe QDs
3.2 Optimization of Experimental Conditions
3.3 Glucose sensing at B(OH)3–TGA@CdTe QDs’ sensor
DLS study shows that the size of particles increases from 7.5 to 33.5 nm when glucose concentrations vary from 0, 1, 3, 5, 10 to 20 mM (Fig S4A), which demonstrates that glucose can promote the aggregation of QDs. TEM images (Fig S4B) support the aggregation of QDs’ probe as well. The aggregation is ascribed to the interaction of diols groups of glucose, boric acid, and carboxylic groups of TGA on surface of QDs. B(OH)3–TGA@CdTe QDs’ sensor produces good reproducibility with a relative standard deviation of 2.1% for ten standard samples each containing 5 mM glucose.
3.4 Selectivity of B(OH)3–TGA@CdTe QDs’ sensor
To test selectivity for the detection of glucose, PL quenching for various PBS containing lactate, fructose, maltose, mannitol, and sucrose was investigated and relative error in glucose (5 mM) is found to be within ±3.0 (Fig S5). Similarly, many amino acids including tyrosine, tryptophan, proline, histidine, glycine, phenylalanine, cysteine, and arginine interfere non-significantly with glucose determination. Experimental results also show that ions SO4 2−, acetate (Ac−), NO3 −, ClO4 −, PO4 3−, Cl−, NH4 +, NO2 −, and F− have negligible interference with determination of glucose. Thus, the method developed in the present study shows high selectivity towards glucose detection over biologically important interfering species.
In summary, we proposed a novel, simple in design, and convenient PL B(OH)3–TGA@CdTe QDs’ system for selective glucose detection simply by mixing and detection protocol. The method presented enables dual modulation for glucose detection based on either intensity change or color change. The visual detection of glucose over a wide range from 1 to 25 mM is particularly attractive, since the method is very simple and most clinically important glucose concentrations are within this range. Moreover, the present method eliminates the need of covalent modification of organic luminescent probes or QDs with organic boronic acids which are generally necessary in photoluminescent glucose sensors.
This project was supported by the National Natural Science Foundation of China (Nos. 21475123 and 21505128), Chinese Academy of Sciences (CAS) and Faculty Development Program of the Bahauddin Zakaryia University, Multan, Pakistan (100 Foreign Scholarships) (No. PF/Cont./2-50/Admin/5398).
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