Silicon quantum-dots-based optical probe for fluorometric detection of Cr 6+ ions

. In this report, silicon quantum dots (SiQDs) with the NH 2 functional group were synthesized with the hydrothermal method. The as-prepared SiQDs exhibit a strong fluorescence emission peak at 441 nm when excited at 355 nm and are effectively quenched upon adding Cr 6+ ions. Hence, SiQDs were used as an optical probe to detect Cr 6+ ions in solutions. The crystal structure of SiQDs was characterized by using X-ray diffraction (XRD). The Fourier-transform infrared spectroscopy (FT-IR) was used to determine the linker groups on the SiQDs surface. The fluorescence spectroscopic technique with an excitation wavelength of 355 nm was used to quantify the Cr 6+ ion concentration in the solutions in the range of 0.1 – 1000 µM. Competition from common coexisting ions, such as K + , Na + , Al 3+ , Zn 2+ , and Pb 2+ , was ignorable. The test with actual samples showed good linearity for the Cr 6+ concentration range of 0.1 – 50 µM.


Introduction
Over the past decades, the increasing toxicity of heavy metals has seriously affected the environment. One of those heavy metals is chromium, which exists in different oxidation states. Trivalent chromium (Cr 3+ ) and hexavalent chromium (Cr 6+ ) are primary oxidation states in the environment. Cr 6+ ions have a stronger impact on human health and the environment than Cr 3+ ions because of its high solubility, carcinogenicity, mutagenicity, and teratogenicity in biological systems [1]. While Cr 6+ exists primarily as highly soluble oxyanions [2], Cr 3+ is less soluble and readily precipitates as Cr(OH)3. Cr 3+ has low toxicity and is considered an essential nutrient for many organisms [3]. In contrast, Cr 6+ is 1000-fold more toxic than Cr 3+ [4]. Nevertheless, the Cr 6+ ion and other Cr 6+ ion forms are used in different industries, including leather tanning, electroplating, painting, and metallurgy [1]. Therefore, chromium contamination has been reported in various industrial sites because of accidental leakages or improper disposal [6][7][8][9]. Hence, the determination of total chromium is crucial for environmental impact studies.
Recently, quantum dots (QDs) have attracted much attention because of their unique physical and optical properties, such as strong absorption, high quantum yield, fluorescence emission, availability for resizing by adding functional groups in the nanoscale, and high stability [10]. However, toxic inorganic precursors are commonly used in synthezing traditional QDs and may harm various biological systems [11].
Among the quantum dots, those of silicon, semiconductor Si nanoparticles ranging from 1 to 10 nm, have attracted much attention in analytical science because of their advantages, such as inertness, nontoxicity, abundance, and low cost [12]. In addition, their highly efficient fluorescence makes them promising optical probes for various biomedical and biological applications [13].
In this paper, the silicon quantum dots ion solutions is shown in the first column of Table   1.
Fluorescence spectra were measured on a spectrofluorometer FL 3-22, Jobin Yvon-Spex, USA. FT-IR spectra were recorded with a wave number range of 375-4000 cm -1 on a Jasco 4600 Fourier-transform spectrophotometer. All the measurements were carried out at ambient temperature.

Results and discussions
The X-ray diffraction pattern of SiQDs is shown in cm -1 appears in the APTES spectrum (Fig. 4b, up) with a weaker intensity than the one in the SiQDs spectrum (Fig. 4b, bottom) because the vibration of the -NH2 bond appears more clearly in SiQDs. or absent (Fig. 5b).
The qualitative result of Cr 6+ ion detection is shown in Fig. 6. The colour images of the samples containing Cr 6+ ion in the range of 1-1000 μM were taken under daylight (Fig. 6a) and UV light with a wavelength of 365 nm (Fig. 6b).
Notably, the fluorescence intensity of the SiQDs solutions decreases remarkably with increasing Cr 6+ concentration. The fluorescence spectra in    The fluorescence intensity of the TL5 sample decreases substantially, corresponding to the most added Cr 6+ amount. The calibration curve is linear for Cr 6+ ion concentrations (R 2 = 0.998) from 0.1 to 50 μM (Fig. 12)

Conclusion
We successfully synthesized SiQDs with surface functionalization. The SiQDs optical probe exhibited high sensitivity towards Cr 6+ ions and was used to detect and qualify actual samples.
The method has good linearity in the range of 0.1-50 µM.