Probing the Effect of Ag 2 S Quantum Dots on Human Serum Albumin Using Spectral Techniques

The understanding of the interaction between protein and quantum dots (QDs) has significant implications for biological applications of QDs. Herein, we studied the effect of Ag2S QDs on human serum albumin (HSA) using UV-Vis absorption spectra and fluorescence spectroscopy and found that the fluorescence intensity of HSAwas gradually decreased with increasing Ag2S QDs concentrations. By using the Stern-Volmer equation for the fluorescence quenching constant (KSV) of the response of Ag2S QDs to HSA as well as thermodynamic equations, the values of thermodynamic enthalpy change (ΔH), entropy change (ΔS), and free energy change (ΔG) were calculated to be −10.79KJ⋅mol, 37.80 J⋅mol⋅K, and −22.27 KJ⋅mol, respectively. The results indicate that Ag2S QDs exert an obvious static fluorescence quenching effect on HSA and electrostatic interaction plays a key role in the binding process. Furthermore, Raman spectral analysis reveals that Ag2S QDs alter the external environment of tyrosine and tryptophan or the C-H bending of HSA but not the α-helical content.


Introduction
With the wide application of QDs in the biological system, their biological effects have attracted increasing attention [1].Up to now, several papers have reported the effects of QDs on proteins, DNA, cells, and so forth.[2][3][4][5][6][7], and obvious biological toxicity of QDs has been detected and confirmed.For example, Shen et al. have studied the interaction between human hemoglobin and CdS QDs by spectroscopic analysis and confirmed that both electrostatic forces and chemical bonds play an important role in the interaction [6].Luo's group investigated nonspecific interactions between QDs and proteins by atomic force microscopy and found that van der Waals forces were the leading forces for the interaction, while electrostatic interactions also played a key role [8].Wang et al. investigated the effects of CdSe/ZnS QDs on human blood serum and found that a hard protein corona was formed by irreversible binding [9].In our recent study, the interaction between CdTe QDs and protamine sulfate has been examined, and a combined dynamic and static quenching model was proposed [10].Li et al. found that TGAcapped CdTe QDs could induce DNA damage and apoptosis of hepatocyte line HL-7702 [11].Xu et al. investigated the toxicity of CdTe QDs capped with three different reagents on engineering Escherichia coli based on protein expression and found that CdTe QDs capped with mercaptoacetic acid have more toxicity than those capped with glutathione and L-cysteine [12].Despite many studies on the interaction between QDs and the biological system, more detailed investigations are still necessary due to effects of many factors such as the size, surface charge, and structure of QDs on their toxicity [13,14].
Ag 2 S QDs are good alternatives for fluorescence detection and imaging of biological molecules due to their excellent properties such as small size, near-infrared emission, and low toxicity [15][16][17].Many previous studies focused on the synthesis and application of Ag 2 S QDs [18][19][20].For example, Li et al. have successfully synthesized near-infrared Ag 2 S QDs with high tumor target ability by injecting Ag 2 S QDs fluorescence probe to mice with tumor and obtained good results in in vivo imaging and dynamic tracking [17].Ag 2 S QDs based fluorescence probes have shown great potential applications not only in in vivo imaging, but also in biomedical research and disease diagnosis [21].These studies have shown potential biological applications of Ag 2 S QDs, but their potential toxicity and effectiveness in biological systems remain to be elucidated, implying the necessity to understand the interaction of Ag 2 S QDs with protein and its mechanism in biomedical applications.
Human serum albumin (HSA) consists of 585 amino acids [22], with a molecular weight of about 66 kD [23], and 17 disulfide bonds which can be folded to form three regions.There are nine annular regions in HSA, with three rings in each region [24][25][26].HSA has an alpha helical structure which can be flexibly reversed to subdomains [27].Due to the presence of tryptophan, tyrosine, and phenylalanine, HSA can emit strong fluorescence under 280 nm excitation [24].Usually, the contribution ratio of fluorescence intensity of phenylalanine, tyrosine, and tryptophan is 0.5 : 9 : 100, indicating that the fluorescence of HSA is mainly derived from its tryptophan residues.Furthermore, the tryptophan residues are sensitive to surrounding environments, which directly leads to changes in the fluorescence intensity of HSA [28].
In the present work, the interaction between Ag 2 S QDs and HSA was investigated using UV-Vis absorption spectra, fluorescence spectroscopy, and Raman spectra.The Ag 2 S QDs were found to have obvious fluorescence quenching effects on HSA at different temperatures (e.g., 297 K, 305 K, and 313 K).The fluorescence quenching mechanism and the thermodynamic mechanism were also systematically investigated.The equipment used included an FLSP920 spectrometer (Edinburgh Instruments), Malvern Zetasizer instrument (Malvern ZEN 3690), FEI Talos F200C transmission electron microscope (FEI Co.), MicroUV-2450 UV visible spectrophotometer (Unocal Shanghai Instrument Co. Ltd.), inductively coupled plasma atomic emission spectrometer (IRIS Intrepid II XSP, Thermo Fisher Scientific, USA), Raman spectrometer (Renishaw, UK) equipped with a confocal microscope (Leica, Germany), DF-101S collector type constant temperature heating magnetic agitator, AL204 electronic balance (Mettler Toledo Instruments Shanghai Co. Ltd.), and GL-21M intelligent high speed refrigerated centrifuge (Changsha Ordinary Instrument Limited Company).

Preparation and Characterization of Ag 2 S QDs. Ag 2 S
QDs were synthesized as previously reported with some modifications [29].Briefly, 0.160 g S powder was mixed with 10 mL hydrazine hydrate in a beaker, followed by stirring the mixture overnight at room temperature.Next, 0.192 g of GSH and 0.043 g of AgNO 3 were dissolved in 25 mL ultrapure water at three flask reactors until the solid was completely dissolved.Under N 2 protection, the solution was supplemented with 0.050 mL of the aqueous S 2− source.After 30 min, the Ag 2 S QDs were obtained at the centrifugal separation speed of 8000 r/min after washing in isopropyl alcohol.The precipitation was redissolved in 30 mL aqueous phase to obtain Ag 2 S QDs solution, which was kept in a refrigerator at 277 K until use.The size and concentration of Ag 2 S QDs were determined by transmission electron microscopy (TEM) and inductively coupled plasma atomic emission spectrometer (ICP-AES).

Investigation of the Interaction between Ag 2 S QDs and
HSA. HSA and different concentrations of Ag 2 S QDs were mixed in a 5.0 mL colorimetric tube in buffer solutions and reacted for 30 min.After that, the HSA fluorescence intensity was recorded at 280 nm.The effects of temperature on the reaction system were investigated by repeating the above steps separately at 297, 305, and 313 K.
The Raman spectra were recorded with inVia micro-Raman spectroscopy.The mixture was composed of HSA and Ag 2 S QDs at concentrations of 1.0 × 10 −5 mol/L and 1.0 × 10 −4 mol/L, respectively.Next, the mixture was dropped onto the foil.After the liquid was dried, the Raman spectra were recorded at the excitation wavelength of 633 nm.

Results and Discussion
3.1.Characterization of Ag 2 S QDs.The size of Ag 2 S QDs was determined by transmission electron microscopy.From Figure 1(a), it can be seen that the average size of Ag 2 S QDs was 3.4 ± 0.2 nm.The top right corner of Figure 1(a) presents the hydrodynamic size distribution images of Ag 2 S QDs, which reveals that the diameter of the QDs is in the range of 3-7 nm with an average size of 4.7 nm.The as-prepared Ag 2 S QDs are consistent with those described in the literature [29].
As shown in Figure 1(b), the UV-Vis absorption spectrum of Ag 2 S QDs was in the absorption range of 250 nm and 300 nm, and there was a small peak centered at 270 nm.The normalized fluorescence emission spectrum of resultant Ag 2 S QDs in aqueous solution was also displayed in Figure 1(b).When these Ag 2 S QDs were excited under UV irradiation with 450 nm excitation wavelength, the maximum emission peak at 700 nm was observed.residues.When combined with Ag 2 S QDs, the interaction led to alterations in the external environment and structure of the protein, resulting in the decrease of fluorescence intensity.With the increase of Ag 2 S QDs concentration, the HSA peak shape and the maximum emission peak remained unchanged.

Investigation of the Quenching Mechanism.
The dynamic quenching only affects the fluorescent molecules and does not change the UV-Vis absorption spectra of fluorescent materials.The UV-Vis absorption spectra of fluorescent materials are changed due to static quenching, which results from the formation of complexes [7,21].Based on this theory, if the UV-Vis absorption spectra of HSA and the complexes of QDs and HSA vary with each other, the quenching mechanism should be static quenching [30].The UV-Vis absorption spectrum after reaction was recorded between 250 and 400 nm (Figure 3(b)).The sum of the absorption spectrum intensity of HSA (Figure 3(d)) and the absorption spectrum intensity of Ag 2 S QDs (Figure 3(c)) is shown in Figure 3(a).With the addition of Ag 2 S QDs, the absorption spectrum of the mixture was different from that of the sum of the individual spectra of both Ag 2 S QDs and HSA, which can be attributed to the interaction between QDs and protein, demonstrating that HSA fluorescence quenching by Ag 2 S QDs is a static procedure.

The Effect of Temperature.
Basically, fluorescence quenching can be divided into static and dynamic quenching.In static quenching, the quencher reacts with the ground-state fluorescent molecule and forms a complex, while in dynamic  quenching only the excited state fluorescent molecule interacts with the quencher [30,31].Temperature has a great influence on the quenching process.The decrease of temperature not only reduces the stability of the ground-state compounds, but also reduces the static quenching degree.However, with the increase of temperature, molecular motion will be accelerated due to the decrease of solution viscosity, leading to an increased intermolecular collision rate and fluorescence quenching [32].
The fluorescence quenching data were analyzed by the Stern-Volmer equation: where  0 and  are the fluorescence intensities of HSA in the absence and presence of Ag 2 S QDs, respectively. SV is the Stern-Volmer quenching constant, which is a measure parameter of the quenching efficiency [33].  is the biomolecular quenching constant. 0 is the lifetime of the fluorescence in the absence of quencher [34], and [] is the concentration of Ag 2 S QDs.As shown in Figure 4, when the temperature increases, the quenching efficiency decreases obviously, suggesting that static quenching plays a dominant role in this quenching process.The data of Table 1 indicate that the fluorescence quenching constant  SV of the interaction of Ag 2 S QDs with HSA decreased with the increase of temperature (pH = 7.4).
According to  SV , when pH = 7.4,  SV is 10 3 orders of magnitude, and the average fluorescence lifetime of biological macromolecules is 10 −8 s [6], so   is larger than the largest diffusion constant.This further confirms that the quenching effect of Ag 2 S QDs on HSA is static quenching [34].

The Fluorescence Lifetime.
Fluorescence lifetime measurement can provide useful information about the type of molecular interactions, and it is the most effective method to distinguish static and dynamic quenching [10].Figure 5 shows the fluorescence decay curves of HSA in the absence and presence of Ag 2 S QDs.The curves are well fitted with a biexponential function and the average lifetime can be calculated by where  1 represents the shorter lifetime and  2 represents the longer one;  1 and  2 indicate the amplitudes of fast and slow decay components, respectively.In order to further verify that the quenching is static but not dynamic, we measured the fluorescence lifetime of HSA in the absence and presence of Ag 2 S QDs.The fluorescence lifetime of HSA without Ag 2 S QDs was 5.63 ± 0.09 ns, and after the addition of Ag 2 S QDs into HSA the fluorescence lifetime was 5.64 ± 0.12 ns.Similar fluorescence lifetime indicated that the quenching effect of Ag 2 S QDs on HSA is static.

Thermodynamic Parameters.
In order to characterize the binding mode of HSA to Ag 2 S QDs, the thermodynamic parameters were analyzed.The standard enthalpy change The free energy change (Δ  ) was estimated from the following relationship: where  is the gas constant, 8.314 J mol −1 K −1 ;  is the temperature (K);   is the equilibrium constant at the corresponding temperature, which stands for the static quenching constant ( SV ) in the present paper [10].According to (3) and ( 4) for plotting the ln   ∼ (1/) diagram [33], the value of Δ  was calculated, and then Δ  and Δ  were estimated based on the slope of the curve (Table 2).
From Table 2, it can be seen that when pH = 7.4, Δ  is a negative value (−10.79KJ⋅mol −1 ) and Δ  is a positive value (37.80 J⋅mol −1 ⋅K).With the increase of temperature, Δ  only has a slight change.In these experiments, Ag 2 S QDs-HSA complexes underwent both negative enthalpy changes (Δ  ) and positive entropy changes (Δ  ) (Table 2), demonstrating that the binding interactions are entropically driven.The negative sign for Δ  indicates the spontaneity of the binding of Ag 2 S QDs onto HSA.According to the literature [33,35,36] and based on the characteristic signs of the thermodynamic parameters during various interactions, the positive enthalpy changes and their values generally represent hydrophobic interaction as the main force.The positive Δ  and negative Δ  indicate that the electrostatic interaction plays major roles in the reaction.Both negative Δ  and Δ  mean that the main force is van der Waals and hydrogen bond interaction.Therefore, from Table 2, it can be seen that the main force between Ag 2 S QDs and HSA is electrostatic interaction.

Conformational Changes of HSA Induced by Ag 2 S QDs.
Raman spectrum is a good tool to study the interaction of protein and nanoparticles [37].Figure 6 shows the Raman spectrum of HSA in the absence and presence of Ag 2 S QDs.The 1655 cm −1 band is attributed to amide I, which is characteristic of high -helical content in HSA.There is no change in the intensity at 1655 cm −1 after HSA interaction with Ag 2 S QDs, indicating that Ag 2 S QDs do not change the -helical content in HSA.A previous study has revealed that the ratio of the intensity at 850 cm −1 to that at 830 cm −1 is related to the environment of tyrosine, showing the exposure of hydrophobic groups of HSA [38].In the present study, the intensity ratio of 850 cm −1 to that of 830 cm −1 changed from 1.34 to 1.17 after the interaction of HSA with Ag 2 S QDs, indicating that the external environment of tyrosine is changed.The 1343 cm −1 band assigned to tryptophan or C-H bending is decreased after HSA interaction with Ag 2 S QDs.Thus, the presence of Ag 2 S QDs can induce alterations in the external environment and structure of HSA.

Conclusions
This work has explored the interaction between Ag 2 S QDs and HSA, and the results show that the fluorescence quenching effect is enhanced with increasing Ag 2 S QDs concentrations, but the enhancement decreases when Ag 2 S QDs on the Fluorescence of HSA.As shown in Figure 2, with the steady increase of Ag 2 S QDs concentration and the decrease of HSA fluorescence intensity, Ag 2 S QDs showed strong quenching effects on the fluorescence of HSA.HSA molecules can emit strong intrinsic fluorescence due to the presence of tryptophan

Figure 1 :Figure 2 :
Figure 1: (a) The TEM image of Ag 2 S QDs (the inset shows the hydrodynamic size distribution of Ag 2 S QDs).(b) The UV-Vis absorption spectrum and the fluorescence emission spectrum of Ag 2 S QDs.
2 S QDs concentration attained saturation.By using UV-Vis absorption spectra, fluorescence lifetime, Stern-Volmer equation, and thermodynamic equations, we have obtained the Ag 2 S QDs and HSA binding fluorescence quenching constant of reaction ( SV ), the thermodynamic enthalpy (Δ  ), entropy (Δ  ), and free energy change (Δ  ) values.The results demonstrate the mechanism of static quenching and spontaneous reaction between Ag 2 S QDs and HSA as well as their interactions mainly as electrostatic attraction.The Raman spectrum shows that Ag 2 S QDs do not change the -helical content in HSA but alter the external environment of tyrosine and tryptophan or the C-H bending of HSA.The integrated data demonstrate that Ag 2 S QDs are a good fluorescence quencher towards proteins such as HSA and they have the potential for the development of ratiometric fluorescent probes.

Table 1 :
Fluorescence quenching constant  SV values of the interaction between Ag 2 S QDs and HSA at different temperatures calculated by the Stern-Volmer equation.

Table 2 :
The thermodynamic parameters of the interaction between Ag 2 S QDs and HSA.