Water Soluble Silicon Nanoparticles as a Fluorescent Probe for Highly Sensitive Detection of Rutin.

In this work, water-soluble fluorescent silicon nanoparticles (SiNPs) were prepared by one-pot hydrothermal method using 3-(2-aminoethylamino)propyldimethoxymethylsilane (AEAPDMMS) as a silicon source and amidol as a reducing agent. The prepared SiNPs showed bright green fluorescence, excellent stability against photobleaching, salt tolerance, temperature stability, and good water solubility. Due to the internal filtration effect (IFE), rutin could selectively quench the fluorescence of the SiNPs. Based on such phenomena, a highly sensitive fluorescence method was established for rutin detection. The linear range and limit of detection (LOD) were 0.05-400 μM and 15.2 nM, respectively. This method was successfully applied to detect rutin in the samples of rutin tablets, Sophora japonica, fry Sophora japonica, and S. japonica carbon with satisfactory recovery.

Introduction

Water-soluble silicon nanoparticles (SiNPs) are emerging small-sized fluorescent silicon nanomaterials. They are mainly obtained by the reaction of a silicon source and reducing agent. Methods of synthesizing water-soluble SiNPs included the water bath method,[1,2] room temperature stirring method,[3,4] microwave-assisted method,[5,6] post-modification method,[7,8] and one-pot hydrothermal method.[9,10] In recent years, the one-pot hydrothermal method has been widely applied to the synthesis of water-soluble SiNPs owing to its superiority of simple operation, inexpensive equipment, and excellent optical properties of the synthesized materials. Compared with traditional fluorescent quantum dots, water-soluble SiNPs are simple in composition and free of heavy metal elements. SiNPs are non-toxic because they can be metabolized in the body as orthosilicic acid and then excreted in the urine. Significantly, SiNPs possessed excellent optical properties, robust stability, and outstanding biocompatibility. Owing to the simple synthesis and excellent properties, SiNPs were considered as ideal fluorescent probes in the fields of analytical science. Lately, fluorescent probes based on SiNPs were successfully used to detect heavy metal ions,[11,12] dopamine,[5] amino acids,[13] vitamins,[6] nitro explosives,[14] etc. Furthermore, SiNPs could also be used for pH sensing and bioimaging.[7,15−19] Nevertheless, the application of SiNPs in the detection of active ingredient of Chinese medicine is relatively rare.

Rutin is the main active ingredient of Sophora japonica. Pharmacological research displayed that rutin can maintain vascular elasticity, enhance resistance, and is effective in anti-inflammatory, antioxidation, antitumor, and antiviral aspects.[20−23] Because of its rich raw materials, high content, and a variety of pharmacological effects, rutin has been widely investigated. In recent years, studies on the role of rutin in modern diseases have shown that rutin is effective in treating diseases such as obesity, gout, diabetes, and cardiovascular diseases.[24−26] Furthermore, rutin can also be used as health care products and a natural food pigment. Rutin as a therapeutical drug has received considerable interest in clinical chemistry and human health owing to its outstanding biological activity. Nevertheless, excessive intake of rutin can cause gastrointestinal discomfort and other physical diseases. Therefore, simple and sensitive detection of rutin in pharmaceuticals is of crucial significance. Currently, several analysis methods including high-performance liquid chromatography,[27] electrochemical analysis method,[28−30] chemiluminescence method,[31] and UV–Vis spectrophotometry[32] have been reported for rutin detection. Most of these methods need complex operation, expensive instruments, and time-consuming operation. The fluorescence analysis method has the advantages of convenient operation, low cost, and outstanding selectivity and sensitivity.[33−35] Up to now, the fluorescence analysis method for rutin detection has also been reported. For instance, Sun et al. fabricated molybdenum disulfide quantum dots through a one-step hydrothermal synthesis method for rutin detection with a linear range of 0.5–32 μM.[36] Sinduja and John synthesized fluorescent carbon dots using asparagine and used them for sensitive detection of rutin in the concentration range of 0.5–15 μM.[37] Lin et al. constructed a dual-emission ratiometric fluorescence probe to selectively detect rutin by the electrostatic interaction of PVP-CuNCs and Rh6G, and the detection limit was 0.84 μM.[38] The above reports provided effective fluorescence methods for rutin detection. Nevertheless, some disadvantages of complicated fluorescent probe preparation process, heavy metals use, narrow linear range, and insufficient sensitivity still existed. Thus, it is of great significance to develop an environment-friendly fluorescent material for highly sensitive detection of rutin.

Herein, by using amidol as a reducing agent and 3-(2-aminoethylamino)propyldimethoxymethylsilane (AEAPDMMS) as a silicon source, novel water soluble SiNPs with green fluorescence were prepared through a one-pot hydrothermal method (Scheme ). The method did not require any post modification and avoided time-consuming multistep synthesis processes. The synthesized SiNPs possessed outstanding stability against photobleaching, salt tolerance, temperature stability, and good water solubility. As the concentration of rutin was increased, the fluorescence intensity (FL intensity) of the SiNPs decreased gradually (Scheme ). Hence, a sensitive fluorescence method for detecting rutin was established using the SiNPs as the fluorescent probe. The method was utilized for the determination of rutin content in rutin tablets and three kinds of S. japonica cannon products.

Scheme 1

Schematic of the Synthesis Method of the SiNPs and Their Use for Rutin Detection

Experimental Section

Synthesis Process of the SiNPs

1 mL of AEAPDMMS was mixed with 4 mL of deionized water. Then, 10.0 mg of amidol was added and stirred continuously for 20 min. Subsequently, the above solution was heated in a Teflon-lined reactor at 200 °C for 8 h. After cooling down to room temperature, the SiNP solution was purified in a dialysis bag with a molecular weight cutoff of 1000 Da for about 3 h. The obtained SiNP solution was stored in a refrigerator at 4 °C for further use.

The SiNPs as the Fluorescent Probe for Rutin Detection

8 μL of SiNP solution was added to PBS solution (10 mM, pH 8.0) and evenly mixed. Then, different volumes of rutin solution were added to the above solution to prepare a series of solutions with different concentrations (the total volume was 2.0 mL). The fluorescence emission spectra were measured after 1 min incubation. The working curve was drawn by the log(F0/F) value of the SiNPs and the concentration of rutin (F0 and F are the FL intensity of the SiNPs before and after adding rutin, respectively). The selectivity experiment was carried out by introducing potential interferences instead of rutin under the same conditions. Both rutin and interfering substances were added simultaneously for the anti-interference experiments.

Analysis of Rutin in Actual Samples

First, rutin tablets, S. japonica, fry S. japonica, and S. japonica carbon were ground into powder, respectively. Then, 0.1, 0.5, 1.0, and 1.0 g of the above four sample powders were weighed and dissolved in 10 mL of methanol, respectively. After ultrasonic treatment for 30 min, the supernatant was collected and centrifuged at 16500 rpm for 20 min. The obtained sample solutions were diluted and the concentrations of rutin in the sample solutions were detected according to the plotted working curve.

Results and Discussion

Optimization of Parameters in the SiNP Synthesis

As illustrated in Figure S1A (Supporting Information), the FL intensity gradually increased with the increase concentration of amidol, and then the FL intensity slightly decreased when the dosage was greater than 10.0 mg. The result in Figure S1B (Supporting Information) indicated that the FL intensity increased significantly when the temperature increased from 120 to 200 °C. As seen in Figure S1C (Supporting Information), the FL intensity gradually increased with the reaction time increased to 8 h. Therefore, 10.0 mg, 200 °C, and 8 h were chosen as the optimal reducing agent dosage, reaction temperature, and reaction time, respectively. As displayed in Figure S2 (Supporting Information), amidol or AEAPDMMS alone produced negligible fluorescence. Significant FL intensity was observed only when amidol and AEAPDMMS co-existed, indicating that the fluorescence primarily came from the reaction products of amidol and AEAPDMMS.

Characterizations of the SiNPs

As shown in Figure A, the SiNPs had a spherical shape and uniform dispersion. The particle size of the SiNPs ranged from 1.29 to 2.69 nm, with an average diameter of 1.81 nm (Figure B). The surface structure of the SiNPs was characterized by FT-IR (Figure C). The characteristic peaks at 3379 and 3295 cm–1 were attributed to the stretching vibration of O–H and N–H, respectively.[39] The absorption peaks at 2929 and 2876 cm–1 represented the stretching and bending vibrations of C–H on the saturated carbon, respectively.[40] The absorption peaks at 1576 and 1477 cm–1 were the stretching vibration of the aromatic ring.[3] The signals at 1314 and 1261 cm–1 belonged to the stretching vibration of C–N and C–O, respectively.[13] The characteristic peak at 1090 cm–1 was assigned to the extensional vibration of Si–O–Si,[41,42] while the peak at 910 cm–1 belonged to the Si–N stretching vibration.[3] The absorption peak at 797 cm–1 was assigned to the wagging vibration of N–H.[43] The FT-IR spectra results indicated that the SiNPs possessed hydrophilic groups, especially hydroxyl and amino groups, which enhanced the water dispersibility and stability of the SiNPs. Furthermore, the elemental composition of the prepared SiNPs was further investigated using XPS. The peaks at 100.9, 152.8, 284.7, 398.9, and 530.5 eV corresponded to Si 2p, Si 2s, C 1s, N 1s, and O 1s, respectively (Figure A). Four peaks at 284.1 eV (C–Si), 284.6 eV (C–C/C=C), 285.8 eV (C–N), and 287.9 eV (C–O) were shown in the XPS spectrum of C 1s (Figure B).[39] Three fitting peaks were presented at 399.2 eV (N–Si), 399.7 eV (C–N–C), and 400.1 eV (N–H) in N 1s XPS spectrum (Figure C).[3] Three peaks were presented in the XPS spectrum of O 1s (Figure D). Among them, two peaks at 531.1 and 531.9 eV corresponded to O–Si and the peak at 532.2 eV represented O–C.19 The XPS spectrum of Si 2p (Figure E) with peaks at 101.5, 102.1, and 102.6 eV corresponded to Si–C, Si–N, and Si–O, respectively.[44] The above results indicated that the XPS determination on the surface constitution of the SiNPs agreed with the FT-IR results. The optical properties of the SiNPs were evaluated by UV–Vis spectrum and fluorescence spectrum (Figure F). Under excitation at 433 nm (curve a), the fluorescence emission peak of the SiNPs was located at 507 nm (curve b). The characteristic absorption peaks at 238 and 426 nm could be assigned to π–π* transitions of C=C (curve c).[3] The solution of the SiNPs showed bright green fluorescence under a 365 nm UV lamp (insert in Figure F). The absolute quantum yield of the prepared SiNPs was 12.08%. Additionally, as seen in Figure S3 (Supporting Information), the fluorescence emission peak was not shifted with the continuous change of excitation wavelength, which indicated that the fluorescence emission behavior was independent of size.

Figure 1

(A) TEM image of the SiNPs; (B) corresponding size distribution histogram; (C) FT-IR spectra of the SiNPs.

Figure 2

XPS spectra of the SiNPs. (A) Full range; (B–E): C 1s, N 1s, O 1s, and Si 2p spectra; (F) fluorescence excitation spectra (a), emission spectra (b), and UV–Vis absorption spectra (c) of the SiNPs (λex = 433 nm, λem = 507 nm).

Stability of the SiNPs

The stability of the SiNPs under different conditions was investigated. The result is shown in Figure . The fluorescence of the SiNPs was relatively weak under acidic conditions. With the increase of pH value, the FL intensity of the SiNPs first increased and then tended to be stable in the pH range of 8–11 (Figure A). Furthermore, the SiNPs showed good reversibility between the pH of 4.0 and 8.0 (Figure S4A, Supporting Information). The zeta potentials of the SiNPs under different pH values (4.0, 6.0, and 8.0) were – 0.41, −4.04, and −9.34 mV, respectively (Figure S4B, Supporting Information). The negative charge of the SiNPs obviously increased with the increase of pH, which indicated that the pH sensitivity of the SiNPs (pH range: 4.0–8.0) was related to the protonation and deprotonation of surface functional groups (hydroxyl and amino). These groups can be protonated and deprotonated at different pH environments, resulting in the change of FL intensity. In Figure B, the FL intensity of the SiNPs remained remarkably stable even when the concentration of NaCl solution was increased to 100 mM. As illustrated in Figure C, the FL intensity of the SiNPs changed only slightly when the temperature was increased from 5 to 85 °C. Furthermore, as seen in Figure D, the SiNPs showed outstanding stability after continuous ultraviolet irradiation at 433 nm for 60 min. The above results indicated that the prepared SiNPs possessed pH stability at pH 8.0–11.0, excellent salt tolerance, thermostability, and antiphotobleaching resistance.

Figure 3

Normalized FL intensity of the SiNPs at different pH levels (A); after addition different concentrations of NaCl (B); after incubation at different temperatures for 10 min (C); FL intensity of the SiNPs at 433 nm of ultraviolet radiation (D).

Optimization of Detection Conditions for Rutin

To explore the optimal detection condition, the influence of the solution pH and response time on the detection was investigated. As was previously described, the FL intensity of the SiNPs was basically unchanged at pH 8.0–11.0. Furthermore, rutin showed the highest fluorescence quenching efficiency for the SiNPs at pH 8.0 (Figure S5A, Supporting Information). Therefore, 8.0 was selected as the optimal pH value in the following experiments. The effect of the reaction time is shown in Figure S5B (Supporting Information). The fluorescence quenching was very fast when the SiNPs were mixed with rutin solution (50 μM). Even when the concentration of rutin was 0.05 μM (Figure S6, Supporting Information), the FL intensity of the SiNPs remained basically unchanged after 30 s. Therefore, 1 min was chosen as the ideal reaction time.

Sensitivity of the SiNPs for Rutin Detection

As displayed in Figure A, significant fluorescence quenching of the SiNPs could be observed when different concentrations of rutin were added to the SiNPs solution. A good linear behavior existed between the log(F0/F) and the concentration of rutin of 0.05–400 μM (Figure B). The LOD was calculated to be 15.2 nM based on 3 s/k (s and k are the standard deviation of the blank solution and the slope of the calibration curve, respectively). Based on the above results, the SiNPs could be used as fluorescence sensor for quantitative detection of rutin. Compared with previously reported methods for rutin detection, the method in this work exhibited a wide linear range and low LOD (Table ).[27,28,31,32,37,38,45] Moreover, the proposed method is simple, rapid, low cost and high sensitivity.

Figure 4

(A) Fluorescence responses of the SiNPs upon addition of various concentrations of rutin. Inset is the photographs of the SiNPs without (left) and with (right) rutin under 365 nm UV light. (B) Fitting curve between the log(F0/F) of the SiNPs and the rutin concentration in the range of 0.05–400 μM.

Table 1

Comparison of the Reported Methods for Rutin Sensing

applied material	method	response time (min)	linear range	LOD	ref
	HPLC		10–500 μg/mL	0.46 μg/mL	(27)
Mg-Al-Si@PC nanocluster	electrochemical	3	1–10 μM	0.01 μM	(28)
Luminol-KIO4-ZnSe QDs	chemiluminescence		8.0 × 10–11-8.0 × 10–7 g/mL	1.1 × 10–11 g/mL	(31)
AgNPs	UV–Vis spectrometry		0.3–4.9 μg/mL	0.09 μg/mL	(32)
CDs	fluorescence		0.5–15 μM	0.1 μM	(37)
PVP-CuNCs-Rh6G	fluorescence	2	5–300 μM	0.84 μM	(38)
BSA-SiNPs	fluorescence	5	0.33–30.33 μM	0.047 μM	(45)
SiNPs	fluorescence	1	0.05–400 μM	15.2 nM	this work

Selectivity of the SiNPs for Rutin

To investigate the selectivity of the SiNPs toward rutin, a variety of potential interfering substances (including common inorganic ions, amino acids, vitamin C, maltose, glucose, fructose, lactose, starch, citric acid, gallate, and urea) were studied. As seen in Figure , no obvious fluorescence quenching could be seen when the above interferences were mixed with the SiNP solution. After that, the anti-interference capability was also investigated by adding the mixture of rutin and interferences into the detection system. The effect on the FL intensity was negligible compared to that on the system only with rutin. The above results showed that the method possessed good selectivity and anti-interference ability.

Figure 5

(A) Fluorescence responses of the SiNPs toward 200 μM interference ions (blue bars) and the subsequent addition of 50 μM rutin (orange bars). (B) Fluorescence responses of the SiNPs toward 200 μM organic interferents (blue bars) and the subsequent addition of 50 μM rutin (orange bars).

Additionally, quercetin, which exhibits very similar structure and UV–Vis absorption spectrum with rutin, may interfere the rutin detection. The response of quercetin to the SiNPs was studied. As seen in Figure S7 (Supporting Information), a slight quenching response was observed for quercetin under the same condition, which has certain interference to the detection of rutin.

Possible Response Mechanism of the SiNPs to Rutin

The mechanism of fluorescence quenching of the SiNPs in the presence of rutin was investigated. First, the fluorescence spectra of the SiNPs and UV–Vis absorption spectra of rutin were investigated. In Figure A, it could be seen that the UV–Vis absorption spectra of rutin overlapped with the fluorescence excitation spectra of the SiNPs. Therefore, the internal filtration effect (IFE) probably occurred in the quenching process. Moreover, the fluorescence decay traces of the SiNPs were detected in the absence and presence of rutin (Figure S8, Supporting Information). The fluorescence lifetime curves of the SiNPs and the mixture of the SiNPs and rutin solution fitted the double exponential function, respectively. The result in Table S1 (Supporting Information) showed that the fluorescence lifetime of the SiNPs was not changed significantly with the addition of different concentrations of rutin. These results further indicated that the quenching mechanism of the SiNPs by rutin was IFE. In view of this, the role of the IFE in the quenching process was further studied. On the basis of the influence of the cuvette geometry on the absorption characteristics and the fluorescence detection of the aqueous solution of rutin and the SiNPs, the IFE was estimated with eq :[46]where Fobsd is the observed maximum FL intensity; Fcor is the corrected maximum FL intensity after removing the IFE from Fobsd; Aex and Aem are the absorbance at the maximum excitation wavelength (λ = 433 nm) and emission wavelength (λ = 507 nm), respectively. As illustrated in Figure S9 (Supporting Information), g is the distance between the excitation beam edge and the cuvette edge (g = 0.25 cm); s is the excitation beam thickness (s = 0.50 cm); d is the cuvette width (d = 1.00 cm). The g, s, and d values depend on the geometry of the measurement. The maximum correction factor (CF) should not exceed 3;[47] otherwise, the correction is unconvincing. The CF of the IFE at each concentration of rutin was calculated according to eq . Table S2 (Supporting Information) showed the CF of the IFE and the relevant parameters. Eobsd and Ecor are the observed and corrected fluorescence quenching efficiencies of the SiNPs after adding different concentrations of rutin, respectively. After calculation, the quenching efficiency was up to 86.84%, which indicated that the fluorescence quenching mainly came from the IFE.

Figure 6

(A) Fluorescence excitation (a) and emission spectra (b) of the SiNPs and UV–Vis absorption spectra of rutin (c). (B) UV–Vis absorption spectra of the SiNPs and rutin as well as the theoretical and experimental spectra of the mixture of the SiNPs and rutin.

In addition, the static quenching effect (SQE) and dynamic quenching effect (DQE) were also investigated through the Stern–Volmer equation after removing the IFE (eq ):[48]where Ksv and [Q] represent the Stern–Volmer constant and the concentration of rutin, respectively; F0 and F are the steady-state FL intensities without and with the quencher rutin. As illustrated in Figure S10 (Supporting Information), no linear relationship existed between the corrected FL intensity ratio (Fcor,0/Fcor) of the SiNPs and the concentrations of rutin, indicating that SQE and DQE were negligible.

The theoretical absorption spectra of the mixture of the SiNPs and rutin were obtained based on the absorbance additivity of Lambert–Beer’s law. As can be seen from Figure B, the experimental absorption spectra of the SiNPs upon addition of rutin were inconsistent with the theoretical absorption spectra. When the SiNPs were mixed with rutin, a new peak was formed at 405 nm. This indicated that the ground state complex may be formed between the SiNPs and rutin, which also contribute to the quenching process. In order to verify this hypothesis, the zeta potential experiment was conducted first. As shown in Figure S11A (Supporting Information), the zeta potentials of the SiNPs, rutin, and the mixture of the SiNPs and rutin solution were −9.34, −8.17, and −11.3 mV, respectively. The above results showed that no electrostatic interaction existed between the SiNPs and rutin. Then, the FT-IR spectra of the SiNPs, rutin, and the mixture of the SiNPs and rutin were determined. As seen in Figure S11B (Supporting Information), after the mixing of the SiNPs with rutin, the O–H stretching vibration signal of the SiNPs decreased at 3379 cm–1, and the N–H stretching vibration signal shifted from 3295 to 3276 cm–1. The absorption peak intensity of rutin at 1657 and 1601 cm–1 decreased and shifted to 1641 and 1576 cm–1, respectively. The results suggested that hydrogen bonds might be formed between the SiNPs and rutin. To further confirm this, the interactions between the SiNPs and rutin were calculated by the Gaussian 16 software package. From the perspective of structure, the SiNPs are rich in hydrogen bond (HB) donors. The long aminoalkyl chain is hydrophobic, which is less likely to expose to aqueous solution. Therefore, we think the carbonyl O atom of the flavonoid moiety could form HBs with the HB donor in the SiNPs. In order to reduce computation cost, the SiNPs were divided into oligomers, further removing the aminoalkyl group, resulting in the SiNP model (Figure S12, Supporting Information). The carbonyl O atom of the flavonoid moiety can form HBs with either the RNH-C6H3R-OH moiety or RNH-SiMe2-NHR moiety of the SiNP model, leading to two different kinds of HB complexes. Figure S13 (Supporting Information) displayed the optimized structures of rutin and the SiNP model. Figure S14 (Supporting Information) showed the optimized structures and relative Gibbs free energies (ΔGs) of HB complexes. From Figure S14 (Supporting Information), it can be seen that rutin can form two kinds of HB complexes with the SiNP model spontaneously (SiNPs model·rutin-1 and SiNPs model·rutin-2). They have comparable relative Gibbs free energies. Hence, hydrogen bonds may be spontaneously formed between the SiNPs and rutin.

To further explore the potential practical application of the established fluorescence method based on the SiNPs, the contents of rutin in rutin tablets, S. japonica, fry S. japonica, and S. japonica carbon were determined, respectively. The detection results indicated that the rutin contents in rutin tablets, S. japonica, fry S. japonica, S. japonica carbon were 327.60, 201.50, 184.42, and 35.85 mg/g, respectively. The recoveries of this method were carried out by spiking a series of known concentrations of rutin. As illustrated in Table , the recoveries varied from 97.20% to 103.87% with relative standard deviations (RSDs) no more than 5.10% (n = 3). The experimental results showed that the established method was reliable for the detection of rutin contents in practical samples.

Table 2

Detection of Rutin in Practical Samples

real samples	initial rutin amount (μM)	spiked (μM)	found (μM)	RSD (%, n = 3)	recovery (%)
rutin tablets	40.25	50.00	90.09 ± 0.91	1.82	99.68
		100.00	137.45 ± 2.17	2.17	97.20
		200.00	245.22 ± 4.79	2.39	102.49
S. japonica	49.52	50.00	99.99 ± 1.47	2.94	100.95
		100.00	148.66 ± 3.57	3.57	99.14
		200.00	249.57 ± 5.29	2.64	100.03
Fry S. japonica	45.31	50.00	94.42 ± 0.86	1.73	98.22
		100.00	146.23 ± 4.05	4.05	100.92
		200.00	253.04 ± 8.51	4.26	103.87
S. japonica carbon	46.96	50.00	97.84 ± 1.82	3.64	101.75
		100.00	144.36 ± 3.36	3.36	97.40
		200.00	246.33 ± 10.21	5.10	99.69

Conclusions

In summary, the water-soluble fluorescent SiNPs with excellent fluorescence were synthesized by a simple one-pot hydrothermal method using AEAPDMMS and amidol as precursors. The prepared SiNPs were used as a fluorescent probe for the detection of rutin with high sensitivity. Experimental results showed that the fluorescence of the SiNPs could be effectively quenched by rutin via the IFE with a wide linear range of 0.05–400 μM and low LOD of 15.2 nM. The method has been successfully applied to the analysis of rutin in the samples of rutin tablets, S. japonica, fry S. japonica, and S. japonica carbon, and the recoveries were 97.20–103.87%. The results showed that the SiNP-based fluorescence probe has high precision for rutin detection and potential application in the detection of rutin in complex samples. Nonetheless, it should be noted that this method also has some limitations. The response of the SiNPs to rutin was mainly due to the IFE mechanism. Some substances exhibit very similar UV–vis absorption spectrum that may interfere in rutin detection. To avoid this phenomenon, we will intend to prepare SiNPs with better selectivity. Moreover, most of the SiNPs reported so far are blue or green fluorescence, and improving the fluorescence of SiNPs to achieve longer emission wavelength is still a major challenge.