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Article Cite This: ACS Omega 2018, 3, 7613−7620
Silicon Quantum Dot-Based Fluorescent Probe: Synthesis Characterization and Recognition of Thiocyanate in Human Blood Debiprasad Roy,† Koushik Majhi,‡ Maloy Kr. Mondal,† Swadhin Kr. Saha,† Subrata Sinha,‡ and Pranesh Chowdhury*,† †
Polymer & Nano Research Laboratory, Department of Chemistry, and ‡Integrated Science Education and Research Centre, Siksha Bhavana, Visva-Bharati University, Santiniketan 731 235, India
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ABSTRACT: Allylamine-functionalized silicon quantum dots (ASQDs) of high photostability are synthesized by a robust inverse micelle method to use the material as a fluorescent probe for selective recognition of thiocyanate (a biomarker of a smoker and a nonsmoker). The synthesized ASQDs were characterized by absorption, emission, and Fourier transform infrared spectroscopy. Surface morphology is studied by transmission electron microscopy and dynamic light scattering. The synthesized material exhibits desirable fluorescence behavior with a high quantum yield. A selective and accurate (up to 10−10 M) method of sensing of thiocyanate anion is developed based on fluorescence amplification and quenching of ASQDs. The sensing mechanism is investigated and interpreted with a crystal clear mechanistic approach through the modified Stern−Volmer plot. The developed material and the method is applied to recognize the anion in the human blood sample for identification of the degree of smoking. The material deserves high potentiality in the field of bio-medical science. amine.22 Recently, the last one is getting more importance22,23,27 because of its structural features. Allylamine moiety possesses the carbon−carbon double bond and the amine group in terminal positions, which are suitable for hydrosilylation28 in the presence of a platinum catalyst at the room temperature. The surface modifier has some effect on the fluorescence property.23 It is found that amine-terminated SQDs are more effective in exhibiting intense fluorescence in comparison to alkyl groups.23,27 Thus, allylamine emerges as the most important surface modifier. Low-level thiocyanate is an important constituent in the human body; but its high concentration exhibits an evil effect. Clinical studies show that the concentration of it in the blood serum becomes high for the smoker in comparison to the nonsmoker.29,30 The concentration of the anion present in the human plasma level found to be in the ranges 9−12 and 2−3 mg/L for the smoker and the nonsmoker,29,30 respectively. The half-life period of thiocyanate in the human blood is 1−2 weeks.31 Therefore, the thiocyanate ion may be used as a biomarker for smokers and nonsmokers.32 During smoking, hydrogen cyanide is formed because of pyrolysis or combustion of nicotine present in tobacco. The inhaled cyanide is detoxified into the thiocyanate through incorporation of mitochondrial rhodanese and β-mercaptopyr-
1. INTRODUCTION Quantum dots (QDs) are highly photostable nanosized (2−10 nm)1 semiconductor having unique fluorescent property, which is due to the quantum confinement effect.2,3 It has a broad range of application in the field of bioimaging,4 optoelectronics,5 photonics,6 biosensing,7 and chemical sensing.8,9 Among the QDs searched so far (CdSe, CdTe, CdS, ZnS, ZnSe, PbS, PbSe, GaAs, GaN, InP, and InAs), silicon QDs (SQDs) are one of the best candidates for fluorescent probe because of low toxicity as well as high photostability.3,10 Apart from noncytotoxicity, SQDs also has tunable fluorescence property, high luminosity,11 and less photobleaching effect12 in comparison with other organic dyes. Moreover, silicon is more abundant compared to others. Therefore, SQDs are a very interesting moiety for research. Various physical and chemical processes such as electrochemical etching,13,14 sol−gel method,15,16 laser ablation,17,18 thermal annealing, thermal vaporization,19 and inverse micelle20−22 are available for the synthesis of the SQDs. Among these, though the inverse micelle is robust one, it has two major drawbacks: (i) easy aerial oxidation at room temperature and (ii) water insolubility. Water dispersibility is one of the most important and essential features required for its analytical and biological research.23 Therefore, surface modification is strictly recommended for prevention of areal oxidation as well as the incorporation of water solubility. Surface modification is done by various materials such as acrylic acid,24 propionic acid,25 vinyl pyridine,26 and allyl© 2018 American Chemical Society
Received: April 28, 2018 Accepted: June 25, 2018 Published: July 10, 2018 7613
DOI: 10.1021/acsomega.8b00844 ACS Omega 2018, 3, 7613−7620
Article
ACS Omega
2.1.2. Fluorescence Study. The steady-state fluorescence spectrum of aqueous ASQDs recorded in ambient temperature is shown in Figure 2. It was found that the material exhibited
uvate (shown in Scheme 1).33 The high concentration of the thiocyanate ion is not only the strongest atherogenic factor but Scheme 1. Detoxification of CN− in the Human Body
also liable for the formation of atherosclerotic plaques within coronary arteries,34 which increases the risk of cerebral infarction stroke.35,36 It is also responsible for increasing the plasma myeloperoxidase level for which protein dialysis may be hampered along with uraemic complication.36 Again, the anion is remarkably responsible for blocking of the iodide uptake into the thyroid gland. This is highly dangerous for children and pregnant women because of the decrease in thyroxin formation and iodide deficiency.37 Among the available methods till date for the thiocyanate sensing, flurometric is the most scientifically convincing, easy, and less time-consuming process. Toida et al.38 reported fluorometeric sensing of the thiocyanate ion using Konig reaction, but a major bottle neck of the method is that there is no concentration-dependent study. Sarkar and his coworker39 studied the same anion sensing using the Cd(II) complex. However, the method is not a robust one. Zhang and his group40 synthesized gold nanoparticles and carbon dot-based probes for thiocyanate ions. As per our best knowledge and literature survey, till date, no report is found for allylaminefunctionalized silicon QD (ASQD)-based thiocyanate sensing. Here, we reported for the first time robust, quantitative, and selective flurometric sensing of thiocyanate ions (a biomarker of smokers and nonsmokers) with the designed and developed ASQD. The indigenous material and method has been tested in real sample analysis.
Figure 2. Steady-state normalized emission spectrum of ASQDs and HSQDs.
emission maximum (λmax) at 439 nm on excitation at 327 nm. The emission of the QDs occurred probably because of the jump of the promoted conduction band electron to the valence band.42 Thus, the fluorescent spectra revealed the successful formation of ASQDs. 2.1.3. Fourier Transform Infrared (FTIR) Study. The FTIR spectrum of ASQDs is displayed in Figure 3. The appearance
2. RESULT AND DISCUSSION 2.1. Characterization of the ASQDs. 2.1.1. UV−Vis Absorption Spectral Study. A steady-state UV−vis spectrum was recorded to study the optical properties of ASQDs (Figure 1). It showed a continuous absorption pattern from 500 to 200 nm with a prominent shoulder around 327 nm. The appearance of the shoulder is a distinctive nature of SQD because it is the corollary of well-studied Γ to Γ direct band gap transition.41 Therefore, the nature and position of the shoulder in the absorption spectra confirmed that ASQDs were synthesized successfully.
Figure 3. FTIR spectrum of the ASQDs.
of a band along with an overtone at 3124 cm−1 (νN−H stretch) and a sharp peak at 1569 cm−1 (νN−H bending) indicated the presence of a primary amine group in ASQD.43 The spectrum also exhibited a band at 1185 cm−1 because of the C−N stretching and another two strong sharp peaks at 1500 and 1396 cm−1 for the vibrational scissoring and symmetric bending of Si−CH244 moieties, respectively. Furthermore, the appearance of a small peak at 1662 cm−1 indicated successful attachment of allylamine in the silicon surface.20,44 Hence, FTIR spectroscopy revealed that ASQDs were synthesized successfully. 2.1.4. Morphological Analysis. The morphological analysis of the synthesized ASQDs was carried out by the assistance of transmission electron microscopy (TEM) (A, B), selected area electron diffraction (SAED) (C) and dynamic light scattering (DLS) (D) study. The TEM image of ASQDs (Figure 4A) reveals that the synthesized QDs were homogeneously dispersed throughout the matrix with almost a spherical shape. It is also evident from the image (Figure 4A,B) that the size of the QDs was uniformly distributed around the 1−2 nm
Figure 1. Steady-state normalized absorption spectrum of ASQD, HTerminated Silicon QD (HSQD), and allylamine. 7614
DOI: 10.1021/acsomega.8b00844 ACS Omega 2018, 3, 7613−7620
Article
ACS Omega
Figure 4. TEM images (A,B), SAED pattern (C), and DLS data (D) of ASQDs.
detection (LOD). The emission spectrum of pure ASQD consists of a sharp peak at 439 nm and a weak band in the region 750−900 nm. The PL intensity at 439 nm was amplified slightly with the gradual addition of the thiocyanate anion up to 1 × 10−6 M (critical point) and then quenched steeply with further increase of the anion (Figure 5b). A similar trend was
range. The presence of prominent lattice fringe and a circular ring pattern in the TEM and SAED images (Figure 4C), respectively, is a sign of the crystalline nature of QDs. The DLS study (Figure 4D) shows that the average size of the QDs was around the 5 nm range. However, the result obtained from the DLS study does not resemble with TEM data (1−2 nm range). This is due to the fact that DLS considers the hydrodynamic size distribution, which is larger than the consideration by TEM. Because the ASQDs were terminated with a hydrophilic group (allylamine), the results obtained by DLS are quite reasonable. Therefore, homogeneous dispersion and uniform size distribution from morphological studies indicate that the synthesized ASQDs are of high quality. 2.1.5. Quantum Yield. The fluorescence quantum yields (φf) of ASQDs in water were calculated with respect to 1methylindole (in aqueous medium = 0.33 and fluorescence excitation wavelength, λ = 280 nm) at 296 K45 by the following equation.46 ϕf = ϕfR
If AR ij n yz2 jj zz IfR A k n R {
Figure 5. (a) PL titration of thiocyanate ions with ASQDs (inset: the expansion of the near-IR region); (b) PL intensity at 439 nm at various concentrations of the thiocyanate ion.
where, φf, If, A, and n represent the fluorescence quantum yield, intensity, absorbance, and refractive index of the solvent, respectively, for the sample under investigation, while φRf , IRf , AR, and nR designate the fluorescence quantum yield, intensity, absorbance, and refractive index of the solvent for reference material, respectively. Both the sample and the reference are excited at the same wavelength and the A, AR values are kept very low (
