1. Introduction

Nanocrystals of semiconducting materials which are also known as quantum dots (QDs) have all three dimensions confined to the 2-15 nm length scale ^[1,2,3]. Quantum dots (QDs) have gained a lot of attention in the past decade. The quantum confinement of their electronic states makes them quite attractive, showing some unique optical properties such as high quantum yield, symmetrical emission spectra, broad-band excitation, photo stability, and readily tunable spectra ^[2,4,5,6] compared to conventional dyes. In recent decades, their applications in cell labeling ^[7]biomolecule detection ^[8], immune reaction analysis ^[5] and drug screening ^[9]have increased remarkably. When QDs are used as biomarkers, especially in vivo bio labeling, the interactions between certain small biomolecules in the bio system and the surface of QDs may influence the efficiency of the electron-hole recombination process, leading to luminescence change of the QDs ^[10].

Interactions between CdSe/ZnS QD and human serum albumin (HSA) ^[4], CdSe/ZnS QD and bovine serum albumin (BSA) ^[11], CdS QD and BSA ^[12], CdS QD and hemoglobin (Hb) ^[13], CdTe QD and HAS ^[14] and CdSe QD and Hb ^[15] have been reported. However, those studies only focus on the interaction of QDs with different proteins, but do not relate the interactions to the specific properties of proteins. The mechanism, which can be utilized to predict the binding of different proteins with QDs, is still obscure. Understanding the mechanism of interaction between nanoparticles and bio macromolecules is important to develop bioengineered nanoparticles as light emitting devices and labeling agents and therapeutic proteins.

Normal hemoglobin levels in blood are 12-16 g/dL in females & 14-18 g/dL in males. Human adult hemoglobin (HbA), A major protein component in erythrocytes is the tetramer of globin chains with two alpha and two beta subunits. The structure-functions relationships of hemoglobin are well-studied. The research onthe interactions of Hb with other molecules such as dimethyl sulfoxide ^[16]anionic amphiphiles ^[17], Ln³⁺ ions ^[18], and hematoporphyrin ^[19] were limited though itis an important functional protein for reversible oxygen carrying and storage. The electrostatic adsorption of ultrafine inorganic particles, such as silica, zirconia, and titanium on Hb have been reported ^[20,21]. This work has examined the physical interactions between bovine Hb and CdTe QDs. The fluorescence quenching constant, binding constant, and conformational changes of Hb during the binding process are reported in the present paper.

Serum Albumin is the most abundant protein in blood plasma having reference level of 3.5-5.0 g/dL. It plays a key role in the transport of large number of metabolites, endogenous ligands, fatty acids, bilirubin, hormones, anesthetics and other commonly used drugs. Bovine serum albumin (BSA) has been one of the most extensively studied protein, particularly because of its structural homology with human serum albumin ^[14]. The BSA molecule is made up of three homologous domains (I, Ⅱ, and ⅡI) which are divided into nine loops (L1-L9) by 17 disulfide bonds ^[22]. BSA has two tryptophans, Trp-134 and Trp-212 that possess intrinsic fluorescence, embedded in the first sub-domain IB and sub-domain ⅡA, respectively ^[23]. BSA is often used as coating reagent to modify the surface of nanoparticles due to its strong affinity towards a variety of nanoparticles, such as gold nanoparticles ^[24], silica nanoparticles ^[25], and QDs ^[26,27]. The BSA modified QDs have been applied to ion sensors ^[28,29], fluorescence resonance energy transfers (FRET) ^[13,30], and chemi-luminescence resonance energy transfer ^[31]. However, the mechanism of interaction between BSA and QDs has not yet been determined, which is often thought as an electrostatic attraction.

Lysozyme is a small monomeric non-globular protein found mostly in tears. The highest values are seen in the age group of 21-40 years, and a decrease in lysozyme concentration observed with an increase in age from 30-40 years. The mean lysozyme content of tears is 1768 μg/ml in healthy subjects. Lysozyme consists of 129 amino acids containing α helix and β sheet domains with 4 disulphide bonds ^[32,33,34]. Lysozyme kills bacteria by rupturing their membrane structures. Lysozyme preferentially hydrolyses the β (1→4) glucosidic linkages between N-acetylmuramic acid and N-acetylglucosaminein the peptidoglycan in bacteria ^[35]. It is used in clinical practiceto treat inflammation, abcess, stomatitis & rheum etc. To further develop lysozyme as a therapeutic agent, the study of its interactions with nanopartilcesare crucial. Recently, interactions of lysozyme with CdTe QDs, TiO₂, SiO₂, ZnO and with silver nanoparticles were reported ^[36,37]. The focus of the study was to understand whether the conformational changes in proteins affect the fluorescence properties of quantum dots and proteins themselves. At the same time, it was interesting to observe how interactions with nanoparticles influence on the protein conformations.

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2. Materials and Method

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2.1. Synthesis of CdTe QDs

Cadmium chloride monohydrate (CdCl₂⋅H₂O), Tellurium powder (200 mesh), thioglycolic acid, 3-mercaptopropionic acid (MPA), Bovin Serum Albumin (BSA), Lysozyme (Lys) and Hemoglobin (Hb) were obtained from Sigma-Alderich. CdTe quantum dots capped with mercaptopropionicacid (MPA) were synthesized by organometallic route as reported by us earlier ^[38]. For CdTe quantum dot synthesis, NaHTe was used as the Te precursor and CdCl₂ as Cd precursor. The freshly prepared NaHTe solution was added to the N₂-saturated CdCl₂ solution containing MPA at pH = 9.0. The molar ratio of Cd²⁺/Te²⁻/MPA was 1:0.2:2.4. The solution was then heated and refluxed under nitrogen flow at 100 °C with continuous stirring. Synthesis of nanoparticles was confirmed by UV-VIS spectroscopy, XRD and TEM.

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2.2. Physical Interactions of QDs & Proteins

Interaction of the CdTe QDs & proteins were studied by fluorescence spectrophotometry at room temperature. The interactions between CdTe QDs (fix concentration of 3 × 10⁻⁷ M) and increasing concentrations of lysozyme (Lys), Bovine Serum Albumin (BSA), Hemoglobin (Hb) (10, 25, 50, 100, 200 µg/ml) werestudied in an aqueous solution. The fluorescence spectra were collected with excitation at 365nm and intrinsic fluorescence spectra of CdTe QDs & proteins were recorded between 450nm to 750nm and 310nm to 410 nm, respectively.

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3. Results andDiscussions

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3.1. Spectral Characterization of CdTe QDs

The absorption spectrum of CdTe QDs shows a broad peak in visible region at 488 nm and the emission peak, λ_Em at 558 nm in Figure 1a, (the excitation wavelength, 365 nm). The emission spectrum of CdTe QDs is in good agreement with reported literature ^[39,40]. The absorptionpeak is well resolved, indicating narrow size distribution of the synthesized QDs. The Transmission Electron Microscopy (TEM) images showed that the size of CdTe QDs was in the range of 3-5 nm (Figure 1d and 1e). The crystalline size calculated using Scherrer equation from XRD data was 3 nm.

Figure 1b shows the X-ray diffraction patterns of the synthesized CdTe QD nanoparticles providesinformation about both the crystal-structure and nanocrystalline properties. The diffraction peaks are broad and weak, but relativeintense peaks at 2θ = 25°, 40°, and 47° correspond well for bulk CdTe ^[39]. The Raman spectrum of aqueous CdTe QDs as shown in Figure 1c ranging from 80 to 400 cm⁻¹ were obtained. In low Raman shift regime between 80 to 200 cm⁻¹ for CdTe QD dominated by two tellurium-related bands at wavenumbers around 121 and 141 cm^{−1 [40]}.

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3.2. Interactions between Lysozyme and CdTe QDs

In case of lysozyme, steadydecrease in the fluorescence of CdTe QDs with minor bathochromic shift from 540 nm to 545 nm was observed (Figure 2a) and steady increase in the intrinsic fluorescence without any peak shift was observed in CdTe-Lysozyme mixtures (Figure 2b). Steady decrease in the fluorescence with minor red shift indicates characteristic change in the 3D structure of lysozyme. Slight bathochromic shift in CdTe QDs fluorescence from 540 nm to 545nm and remarkable increase in intrinsic fluorescence of QDs-lysozyme interaction at lysozyme concentration of 4.0 × 10⁻⁶ M signifies degeneration of hydrogen bonding between alpha helices of lysozyme and possible conformational change.

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3.3. Interactions between Hemoglobin and CdTe QDs

The interactions of Hemoglobin with CdTe QDs interactions showed similar trends as observed in lysozyme-QDs interactions. The steady decrease in fluorescence of CdTe QDs with increase in Hb concentration without any peak shift was observed (Figure 3a) and linear increase in intrinsic fluorescence was observed (Figure 3b).

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3.4. Interactions between BSA and CdTe QDs

The BSA shows increase in both, the QDs fluorescence and intrinsic fluorescence of BSA (Figure 4a). The fluorescence of CdTe QDs increased linearly with increase in concentration of BSAwithout any peak shift in its intrinsic fluorescence (Figure 4b). The steady increase in the fluorescence CdTe QDs after interaction with BSA and intrinsic fluorescence of BSA strongly indicate that some conformational changes are occurring due to the interactionsuggesting availability of more binding sites for CdTe QDs.

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3.5. FTIR Spectrum Analysis

To study the mechanism of electrostatic interactions of CdTe QDs with proteins at different concentrations, FTIR spectra of CdTe QD-protein complex were determined. In case of CdTe QD-lysozyme, Figure 5a at lower concentrations of Lysozyme (10-50 µg/ml) FTIR spectra show the prominentpeaks at 1450cm⁻¹ pertaining to proline (71 & 103) side chain and 1724 and 1796 cm⁻¹ are assigned for (C-N) amides (C=O stretching mode). With increase in concentration of Lysozyme intensity of peak at 1450 cm⁻¹ was increased indicating more interactions of CdTe QDs with proline. As alpha helices are present on the outer side of the lysozyme, possibly prolines from these sites are interacting with QDs.

The Figure 5b shows the FTIR spectra of CdTe QD-Hemoglobin complex. The peaks are observed at 1446, 1722, 1796 cm⁻¹ assigned to stretching vibrations of Amide (C=O stretching mode) and peak at 1651 cm⁻¹ assigned for amine (-C=N- bend) ^[41]. At higher concentration of Hb (200 µg/ml), intense peak at 1651 cm⁻¹ indicates (N-Htrans to carbonyl oxygen) Amide-Ⅱ bend suggesting increased hydrogen bonding between porphyrin ring of Hb and QDs. At lower concentrations of Hb (10-100 µg/ml) FTIR spectra indicate interactions between only (C=O) amide reagion of protein & CdTe QD.

In case of CdTe QD-BSA complex, Figure 5c shows three prominent peaks at 1030cm⁻¹, 940 cm⁻¹ and 814 cm⁻¹. Peaks at 1030cm⁻¹, 940 cm⁻¹ are assigned to streching vibrations of (C-O) Amide-I and (-COOH) side chain of serine amino acid respectively. The peak at 814 cm⁻¹ is assigned to (-C-H) bend of p-substituted aromatic amino acid such as tyrosine ^[41,42]. Furthermore, rise in the intensity of above three mentined peaks, especially the peak at 814 cm⁻¹ with the increasing concentration of BSA (from 10 to 100 µg/ml) indicateincreasing contribution of serine and tyrosine to electrostatic interaction with QDs. Surprisingly, at higher concentration of BSA (200 µg/ml) peak at 814 cm⁻¹ supresses remarkably shows hindered tyrosine interaction with QDs due to conformational changes in BSA. Scheme 1 depicts the locations of amino acids on respective proteins, the sites of interaction with negatively charged CdTe QDs.

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3.6. Stern-Volmer Analysis of CdTe QD-Protein Interactions

The changes in the interactions between CdTe QDs and proteins reflect in the changes of the intrinsic fluorescence of proteins at 355 nm originating from the tryptophan (Trp) and tyrosine (Tyr) residues. These changes in the intrinsic fluorescence of proteins can be analysed by the Stern-Volmer equation and the trends of binding affinities of CdTe QDs and proteins can be understood. The observed fluorescence quenching probably arises from the energy transfer occurring between proteins and CdTe QDs. The intrinsic fluorescence of the proteins was quenched in case of lysozyme and hemoglobin and incresed gradually in BSA. The fluorescence quenching mechanism can be analyzed quantitatively at different concentrations of proteins at constant temparature with the Stern-Volmer equation.

$\frac{F0}{F}=1+Ksv.~\left[ Q \right]$

(1)

where, F₀ and F are the relative fluorescence intensities of protein at 355 nm in the absence and presence of quencher, respectively. K_sv is the Stern-Volmer quenching constant, and [Q] is the concentration of quencher (CdTe QDs).The Stern-Volmer plots of proteins (Lysozyme, Hb, BSA) at different concentrations.

In case of CdTe QD-Lysozyme, Figure 6a shows the linear increase in fluorescence quenching (K_q) up to the concentration of 50 µg/ml and then quenching suddenly decreased linearly up to 200 µg/ml. The increase in K_q indicates more binding affinity of lysozyme to CdTe QDs and its susceptibility to the conformational changes up to its certain concentration (10-50 µg/ml). The further increase in the concentration of lysozyme reduces the binding affinity for CdTe QDs. This may be because of steric hindrance for lysozyme at higher concentration which may be either because of saturation of CdTe QD surface or loss of co-operative binding.Figures 2a and 2balso show gradual increase in intrinsic fluorescence spectra.

However, in case of CdTe QD-Hemoglobin interaction, there was no change in K_q up to 50 µg/ml as shown in Figure 6b and then it subsequently increased than hat have seen in case of BSA and Lysozyme. Relatively unchanged fluorescence at lower concentration of Hb quenching suggests that hemoglobin may be due to the presence of porphyrin ring in hemoglobin which imparts rigidity to the structure. Therefore, at low concentration and very rare chance of conformational change it shows stationary fluorescence quenching then at higher concentration due to structural rigidity steric hindrance may occur against acquiring electrostatic interaction with CdTe QDs because of high affinity towards them.

In case of CdTe QD-BSA, Figure 6c shows gradual decrease in K_q upto the concentration of 50 µg/ml of BSA then it suddenlyincreased at higher concentration of BSA (100-200 µg/ml). It also shows linear decrease in K_q to concentration 25 µg/ml. Slight change in K_q linearity from 25-50 µg/ml due to conformational changes in BSA which were also seen in fluorescence spectra of BSA (Figures 4a and 4b).

Table 1 shows Stern-Volmer equation analysis for each protein. This table represents relation between binding affinity of CdTe QDs with proteins in the terms quenching of CdTe QDs fluorescence. In case of BSA, binding constant is directly proportional to quenching constant. Upto the concentration of 25 µg/ml binding Constant (K_sv = 0.0264) shows gradual increase with Quenching constant also, which marks optimum concentration of CdTe QD for BSA. After that it again shows gradual decrease in binding constant (K_sv = −0.0261 to −0.0005) and quenching constant (K_q = −1.3 to −0.0 3), which may again due to conformational change in three-dimensional structure of BSA.

All three proteins show that binding constant (K_sv) is directly proportional to the fluorescence quenching constant (K_q). Both the proteins Lysozyme and BSA show increase in K_sv with CdTe QDs up to the concentration of 50 µg/ml and 25 µg/ml, respectively. At higher concentration of Lysozyme & BSA, K_sv decreasedsubsequently. In case of Hemoglobin, there is no considerable change in K_sv up to the concentration of 50 µg/ml, after that there is gradual increase in K_sv up to the concentration of 100-200 µg/ml which was also supported by UV-Vis (Figure 3a, 3b) and Stern-Volmer analysis (Figure 6b).

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3.7. Analysis of Raman Spectra

Figure 7 shows Raman spectra of individual CdTe QD-protein complex (He-Ne laser λ_ex = 632.8nm). As shown in the Figure 7a, in CdTe QD-BSA complex, peaks at 1650-1660 cm⁻¹ band regions belong to contribution from amide bond (CO-NH), which is characteristic of high α-helical content in BSA. The 1002 cm⁻¹ band region related to the phenylalanine and 1340-1350 cm⁻¹ band region related to tryptophan or C-H bending.

In case of CdTe QD-Hemoglobin complex in Figure 7b peaks in the region of 150-600 cm⁻¹ characterized by the stretching modes of the Fe ligand bonds and peaks in the region of 600-1200 cm⁻¹ assigned to pyrrole vibrations, 1300-1400 cm⁻¹ region called oxidation state marker band region, 1650-1500 cm⁻¹ region: called core size or spin state marker band region.

These results indicate that each protein interacts specifically with CdTe at the nanoparticle-protein interface. This is significant for the understanding of how nanoparticle-protein interactions will influence their respective physicochemical properties.

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4. Conclusion

In summary, all three proteins, Lysozyme, BSA and Hemoglobin which belong to non-globular, globular and metalloprotein families respectively interact differently with CdTe quantum dots. For these three proteins, a general trend was observed in case of Lysozyme and BSA that at protein concentrations from 10 µg/ml to 200 µg/ml, binding affinity (K_sv) to CdTe QDs is directly proportional to quenching constant (K_q). Amongst relative change in binding affinity (K_sv) of proteins to CdTe QDs is highest in case of BSA & lowest in case of Hemoglobin.

All three proteins have different electrostatic interactions with CdTe QDs reflecting in different binding and quenching constants. Moreover, the concentration of the protein also has the influence on the protein-CdTe QD interactions. We believe that this anomalous fluorescence quenching behavior of all three proteins studied here resembles characteristic three dimensional structures and rigidity of respective proteins and the mechanism of QDs interaction. According to fluorescence spectra and Stern-Volmer equation, among three proteins BSA is structurally unstable after interaction with CdTe QDs andHemoglobin is most structurally stable due to its rigid porphyrin ring structure. In case of Hemoglobin-CdTe QDs interaction, we predict that due to its rigid porphyrin ring structure, there was no change in the fluorescence quenching up to the concentration of 50 µg/ml. After that the sudden rise in quenching suggests conformational change in three-dimensional structure of the protein in such way that more binding sites became available for interaction with CdTeQDs resulting in the increase in fluorescence quenching. Similarly, in case of Lysozyme and BSA, which consist of alpha helices as a major constituent in three-dimensional structure of protein, the conformation may change at a particular concentration and thereafter a significant drop in fluorescence quenching was observed.

In conclusion, this study reflects that nanoparticle-protein interactions depend not only on the surface chemistry of the nanoparticle but also on the protein structure. Each globular, metalloprotein and non-globular proteins interact differently and have variable effects on the fluorescence property of the Quantum dots. At the same time the intrinsic fluorescence of the proteinsis also influenced. The understanding of these interactions is important for in-vitro and in-vivo applications of Quantum dots in the field of biomedical and bioengineering research and advanced biological applications.

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Acknowledgements

AK gratefully acknowledges the funding support from Nano mission, Department of Science and Technology, India. AK is also thankful for the support given by Dr. Satishchandra Ogale, National Chemical Laboratory for characterization of nanomaterials.

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Conflict of Interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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