Synthesis and Characterization of Bovine Serum Albumin-Conjugated Copper Sulfide Nanocomposites

A simple biomolecule-assisted solution route was developed to synthesize Bovine Serum Albumin-conjugated copper sulfide (CuS/BSA) nanocomposites, directly using copper salts and thioacetamide (TAA) as the starting materials with a zwitterionic surfactant Bovine Serum Albumin (BSA) as foaming and stabilizing agent. The CuS/BSA nanocomposites have been characterized by UV, TEM, Zeta, DLS, XRD, and FTIR. The results indicate that the as-prepared CuS/BSA nanocomposites are approximate sphere with a size distribution from 10 to 35 nm in diameter and good dispersibility, depending highly on concentration of BSA concentration. These protein-assisted synthesized nanocomposites have a great potential application in biomedical engineering and microelectronics.

In the past decade, many studies show that biological macromolecules, such as amino acids, proteins, DNA and RNA, are capable of controlling nanomaterials nucleation and growth to various different degrees [16].The obtained biomolecules-conjugated nanomaterials are bioactive and biocompatible, which could provide bioactive functionalities throughout the nanocrystal surface for further biological interactions or couplings and then be used in life sciences for luminescence tagging, drug delivery, and many other aspects [16][17][18].Bovine Serum Albumin (BSA), one of the most widely studied proteins [19], has been frequently adopted to synthesize various nanocrystals.Singh et al. have prepared protein capped Ag [20,21], Au [20], Pt [21], Ag-Au [20], Ag-Pt [21] nanomaterials in aqueous BSA foam and study their cytocompatibility against human gingival fibroblasts.Yang et al. have successfully fabricated BSA-ponjugated Ag 2 S Nanorods [22,23] and HgS nanoparticles [24], and discussed their formation mechanism.Our group firstly reported a simple, environment-friendly, controllable synthetic method of CuSe nanosnakes at room temperature using BSA as foaming agent [25].All these results suggest a possible biomimetic role for BSA as "soft template" in materials chemistry.
Herein, we firstly reported a convenient, simple, nontoxic, and environment-friendly method to obtain CuS nanoparticles in the BSA solution.Our results showed that the synthesized CuS/BSA nanocomposites have controllable sizes and sphere shapes, which lay foundation for investigating further application in different fields.

Experimental
2.1.Materials.Copper nitrate (Cu(NO 3 ) 2 ) and thioacetamide (TAA) were A.R. reagents from Sinopharm Company, China.BSA was of electrophoretic purity, purchased from Xiamen Sanland Chemicals Company Limited, China.The average molecular weight of it is about 68,000 Da.All the abovementioned chemicals were used without any further purification.

Synthesis of CuS
/BSA Nanocomposites.In a typical process, 5 mL of 25 mM Cu(NO 3 ) 2 aqueous solution and 10 mL of 1∼4 mg/mL BSA aqueous solution were mixed with vigorous stirring at room temperature.The mixed solution of the BSA-Cu 2+ emulsion was kept static under nitrogen protection for 2 hours.Then 5 mL of 25 mM TAA aqueous solution were added.Immediately after TAA addition, the solution changed to black, indicating the formation of colloidal CuS particles.The mixed reaction solution was again kept static under ambient conditions for 96 hours and then separated by high speed centrifuging at 15 000 rpm.The collected black solidstate product was washed with double distilled water and ethanol for three times and dried in a vacuum at room temperature for 24 hours.

UV-Vis Measurement.
Figure 1 shows the UV spectra of pure BSA, BSA-Cu 2+ , and BSA-CuS in the 190-400 nm wavelength range.The spectrum of pure BSA showed the special absorption peak at 280 nm.Curve (b) was the spectrum of BSA-Cu 2+ complex, showing no obvious shift of absorption peak at 280 nm and significantly enhancement of absorption before 280 nm, which is probably because the BSA protein can provide multiple binding sites for Cu 2+ , Cu 2+ can react with BSA, and the binding sites may include the -NH, -SH, -COOH, and -OH groups.The spectrum of BSA-CuS showed that the absorption peak shifted to a lower wavelength of about 196 nm after the TAA aqueous solution was added to the BSA-Cu 2+ solution, which indicated that the S 2− released from TAA reacted with Cu 2+ to form CuS/BSA nanocomposites.Curve (c) has a gradually increasing absorbance toward shorter wavelengths in accord with the results in the literature [22].Curve (d) was the spectrum of CuS formed directly from the reaction between Cu 2+ and TAA without BSA, showing broad absorbance at the region of 190-250 nm.This phenomenon is likely due to the overlap of absorption bands, which occur at different energies for nanocrystals having disparate sizes.
Figure 2 shows the UV spectra of CuS/BSA nanocomposites synthesized in different concentration BSA solution.All curves indicated that the special absorption peak of CuS/BSA nanocomposites at the same site (∼190 nm).The absorption intensity and half-peak width are both correlated to the particle size of nanomaterials [26,27].According to this principle, we can conclude that the size of CuS/BSA nanocomposites is the maximum in BSA 1.0 mg/mL, and the size of CuS/BSA nanocomposites is the minimum in BSA 3.0 mg/mL.3(e), 3(f), 3(g), and 3(h) are the higher magnification TEM images.The CuS/BSA nanocomposites are approximately spherical and the particle size is in the range of 10-35 nm.The formed CuS/BSA nanocomposites exhibited some degree of aggregated morphology in lower concentration BSA solution (c < 2 mg/mL).With the increase of BSA concentration, the monodispersity of CuS/BSA nanocomposites was gradually improved, which is attributed to that more and more BSA molecules as stabilizing reagent modify the formed CuS nanoparticles.Analysis of the TEM images in Figures 3(d depending highly on concentration of BSA, which is attributed to that the number of BSA molecule without binding Cu 2+ was increased in the solution.When BSA concentration was 4 mg/mL, the value of positive charge is the maximum about +7.41 mV, we can deduce that the monodispersity of CuS/BSA nanocomposites at this stage is the best.This result is consistent with TEM images.CuS nucleation depends on the number of binding sites of BSA and Cu 2+ ; the amount of Cu 2+ , is fixed, so the number of binding sites is also fixed; with the increase of BSA concentration, the binding sites of BSA are also accordingly increased; within the range of concentration of 2 mg/mL BSA solution, more and more Cu 2+ ions were bound to the binding sites of BSA; therefore, the size of CuS/BSA nanocomposites became smaller and smaller; when the BSA concentration reaches over 2 mg/mL, the number of nucleation does not increase; therefore, the size of CuS/BSA nanocomposites is not changed.In addition, the size of CuS/BSA nanocomposites in 4 mg/mL BSA solution was also determined by dynamic light scattering as shown in Figure 5 nanoparticles, whose average diameter was ∼9 nm, and the water molecule around the nanoparticles forming hydrodynamic radius.Subsequently, we also observed that asprepared CuS/BSA nanocomposites were very stable within 6 months.It can be attributed to BSA as a stabilizing agent to block the further growth of the CuS nanoparticles.

FT-IR Measurement.
To study the formation mechanism of CuS/BSA nanocomposites in the BSA solution, the FT-IR spectra of pure BSA, BSA-Cu 2+ , and BSA-CuS powders were determined.The FT-IR spectra and the data of the main peaks are, respectively, demonstrated in Figure 7.The IR peaks of pure BSA at 3430, 3062, 1652, and 1531 cm −1 are assigned to the stretching vibration of -OH, amide A (mainly -NH stretching vibration), amide I (mainly C=O stretching vibrations), and amide II (the coupling of bending vibrate of N-H and stretching vibrate of C-N) bands, respectively.The difference between the IR spectrum of pure BSA and that of BSA-Cu 2+ is obvious such as the characteristic peak of -NH groups disappearing, suggesting that there might be coordination interaction between Cu 2+ and -NH groups of BSA, which may play an important role in the formation of CuS nanoparticles.In addition, the new peaks of BSA-Cu 2+ at 1021 cm −1 and 824 cm −1 might be contributed to the interaction of Cu 2+ and BSA.The strong peak at 1383 cm −1 in the BSA-Cu 2+ and BSA-CuS spectra is attributed to the absorption of NO 3 −1 , which was introduced by the addition of Cu(NO 3 ) 2 .Comparing the IR spectra of BSA-CuS with those of pure BSA, the characteristic peak of -OH groups shifts to a high wavenumber of about 11 cm −1 and the characteristic peak of -NH groups hardly disappears.The results indicate that there might be conjugate bonds between the CuS nanoparticles and -OH groups and -NH groups of BSA.

The Possible Mechanism.
A possible mechanism for the formation of CuS/BSA nanocomposites is suggested as follows: With a net reaction, The scheme of CuS/BSA nanocomposites formation in the BSA solution is illustrated in Figure 8. Firstly, Cu 2+ (green ball) forms complex with BSA as shown in (1) when Cu(NO 3 ) 2 is added to BSA solution.Secondly, when TAA is added into the solution, the S 2− (yellow ball) combines with Cu 2+ cooperated by BSA to form CuS (2).Finally, some CuS (small blue ball) absorb on or bond to BSA molecules and form CuS/BSA nanocomposites (big blue ball) as shown in (3).The CuS/BSA nanocomposites of 10∼35 nm in diameter depend highly on concentration of BSA concentration, which also prove the formation of CuS/BSA nanocomposites.
BSA protein can provide multiple binding sites for Cu 2+ , Cu 2+ ions can react with BSA, and the binding sites may include the -NH, -SH, -COOH, and -OH groups.Then, comparatively unstable TAA slowly hydrolyzed to release the S 2− ion, which reacted with Cu 2+ ions to form CuS nuclei surrounding some special sites of BSA.With aging, the growth of CuS nanoparticles started on them.As a soft template, BSA is very stable stabilizing agents, and has a great effect on the monodisperse of nanoparticles by BSA concentration dependent.Therefore, the CuS/BSA nanocomposites were obtained.

Conclusions
The monodispersed CuS nanoparticles were successfully synthesized by using BSA as the foaming and reagent.They are approximately spherical, with a size distribution from 10 to 35 nm in diameter and good dispersibility, depending highly on concentration of BSA.These proteinassisted synthesized CuS nanomaterials have a great potential application in biomedical engineering and microelectronics.
CuS/BSA Nanocomposites.UV-vis spectra were measured at 20 • C with a Shimadzu UV-2450 UV-visible spectrophotometer equipped with a 10mm quartz cell.The size and morphology of nanoparticles were obtained by using a JEOL JEM-2010 transmission electron microscope, operating at an accelerating voltage of 200 kV.Zeta potential and Particle size were measured with NICOMP 380ZLS Zeta potential/Particle sizer.These samples were characterized by using Rigaku D/max-2200/PC X-ray diffractometer with monochromated CuKa radiation operated at 40 kV and 100 mA.The Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer Paragon-1000 FTIR Spectrometer.

Figure 3
is the TEM images of as-prepared CuS/BSA nanocomposites in different concentration BSA solution, Figures3(a), 3(b), 3(c), and 3(d) are lower magnification TEM images of CuS/BSA nanocomposites, and Figures

Figure 8 :
Figure 8: Scheme of CuS/BSA nanocomposites formation in the BSA solution.