Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Size controllable, pH triggered reduction of bovine serum albumin and its adsorption behavior with SnO 2 /SnS 2 quantum dots for biosensing application Dhananjayan Nathiya a , Karuppasamy Gurunathan b , Jeyaraj Wilson a,∗ a Department of Bioelectronics and Biosensors, Alagappa University, Karaikudi, 630 003, India b Department of Nanoscience and Technology, Alagappa University, Karaikudi, 630 003, India ARTICLEINFO Keywords: Bovine serum albumin Tin sulfde Hybrid composite Melatonin Biosensor ABSTRACT The pH dependent size control of Bovine Serum Albumin (BSA) and its biosensing potential with Tin Oxide and Tin Sulfde Quantum Dots (SnO 2 /SnS 2 QDs) is reported. The role of disulfde bond cleavage in BSA at acidic and circumneutral pH conditions (3 and 7.2) with sodium borohydride as reducing agent, makes changes in its stability, prone to aggregation and microcapsule formation respectively. Here, the structural reduction in BSA (r- BSA2) at pH 7.2 probed more –OH groups eventually creating adhesive surfaces for SnO 2 /SnS 2 QDs due to electrostatic and hydrophobic forces, thus avoids the aggregation of SnO 2 /SnS 2 QDs in composite formation. The accumulation of more active sites in SnO 2 /SnS 2 QDs decorated r-BSA2 (SnO 2 /SnS 2 @r-BSA2) nanobiocomposite favored suitable orientation for Melatonin (Mel) detection. The designed SnO 2 /SnS 2 @r-BSA2 nanobiocomposite over the screen printing electrode exhibited electrochemical detection capabilities for the selective and sensitive recognition of Mel in the linear range of 0.2–1000 μM with a lower detection limit of 16 nM. Additionally, the fabricated biosensor was successfully validated for the determination of Mel in real samples. 1. Introduction Bovine Serum Albumin (BSA), a heart shaped globular protein, composed of 583 amino acids in a single polypeptide chain with dis- ulfde linkages as well as active functional groups such as carboxyl and thiol groups, plays a major role because of their intensive research activities in recent years. Since the protein model is in high similarity of 76% sequence homology with human serum albumin, widely employed in biomedical applications [1–3]. Interestingly, the critical infuencing factors of BSA such as interaction with other biomaterials/nanomater- ials, identifcation of most important residues, structural changes with varying pH and temperature conditions developed interest to analyze their electrochemical behavior for various applications [4,5]. Firstly, the insight study on structural changes in BSA at diferent pH condition remained to be challenge and its application in biome- dical feld is on trend now owing to its various physiological functions. At acidic pH range (< 3.5), an extended form of structure due to the disulfde bond integrity and aggregation of BSA result to fbril forma- tion. In contrast on increasing pH from neutral (> 7.0), exhibit normal form and some of the disulfde bonds in protein can be cleavable by reducing environment and found to produce micro and nanocapsule formation, whereas all the disulfde bonds are protected between acidic and neutral (pH 4–7) [5,6]. However, the understanding of disulfde bond cleavage and structural deformation in BSA at acidic and neutral pH conditions remains to be a challenge. Secondly, BSA also found to undergo structural changes on adsorption by providing stabilized platform due to the contribution of electrostatic and hydrophobic in- teractions [7,8]. The reported materials that initiated the structural changes in BSA upon adsorption are ellagitannin [9], chitosan [10], tellurium [11], dextran [12], polypyrrole [13], etc. It is well observed from these reports that the structural changes of protein with suitable performances in the composite pave a novel platform to designing new innovative hybrid biomaterials for biosensing application. The ad- sorption and folding capacity of BSA with other biomolecule/nano- materials depends on the charging nature which also gets varied by increasing pH [10,14]. At pH 7, negative nature of BSA bind electro- statically with another negatively charged metal nanoparticle due to the presence of several positively charged lysine amino acid in that [15,16]. In this study, we used sodium borohydride, a well-known strong reducing agent, to investigate di-sulfde bond cleavage in protein that leads to structural changes at pH3 and pH7.2. The change in sec- ondary structure of BSA upon adsorption with nanomaterial and its https://doi.org/10.1016/j.talanta.2019.120671 Received 17 August 2019; Received in revised form 6 December 2019; Accepted 22 December 2019 ∗ Corresponding author. E-mail address: wilson.j2008@yahoo.com (J. Wilson). Talanta 210 (2020) 120671 Available online 23 December 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved. T