Size-dependent interaction of gold nanoparticles with transport protein: A spectroscopic study Smritimoy Pramanik, Paltu Banerjee, Arindam Sarkar, Subhash Chandra Bhattacharya à Department of Chemistry, Jadavpur University, Kolkata 700032, India article info Article history: Received 25 September 2007 Received in revised form 9 April 2008 Accepted 10 June 2008 Available online 21 June 2008 Keywords: Fluorescence Quenching Nanoparticles BSA Adsorption abstract Gold nanoparticles of different sizes have been synthesized using sodium citrate as a reducing agent for tetrachloroauric (III) acid. The formed gold nanoparticles have been characterized by the UV–visible and transmission electron microscopy (TEM) measurements. The different sized gold nanoparticles have been used to study the interaction with model transport protein, bovine serum albumin (BSA). Experimental results reveal that BSA molecules adsorbed on the metallic surfaces, suffer strong quenching of their fluorescence and the rate of quenching efficiency is different for different particle size. The analysis of the quenching results has been performed in terms of the Stern–Volmer equation. The mechanism of quenching of fluorescence has been explained. The extent of adsorption of BSA on the gold nanoparticles has been estimated. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Nanoparticles have been the focus of intensive research, not just because of their size-dependent optical property but also due to their dimensional similarities with biological molecules [1–7]. Development of new materials at the nanometric scale, especially in the case of metals and semiconductors has been the object of intense research for the last few decades [8–13]. The physical and chemical properties of the surface of these nanoscale materials are also attracting increasing attention in catalytic chemistry [14,15]. The use of nanostructured metals in different fields such as medicine is well known and has been studied vastly from different point of views [16–18]. As the dimensions of the particle shrink into the nanometer range, there are significant role of the surface in controlling the overall energy of the particles. Solid clusters in the nanometer size show interesting size-dependent chemical properties. These chemical properties help the chemists for the functionalization of different sized nanoparticle. Conjugates of chromophores and gold nanoparticles can be used as multi labels in simultaneous optical and electron microscopy and as energy-transfer assays for the detection of DNA, [19–23] or proteins, [24] or in optoelec- tronics [25]. Molecular chromophores situated in the vicinity of isolated colloidal metal particles in suspension usually experience quenching of their fluorescence [26–34]. Quantification of protein adsorption on colloids or nanoparti- cles is a challenging problem as small changes in the surface structure can profoundly affect the stability of the colloidal suspension. Thus, the quantitative study of protein interaction with surface is difficult. The quantitative analysis of protein and nucleic acids is quite essential in biochemistry. Fluorescent organic dyes have been employed for this purpose [10,11]. Bovine serum albumin (BSA) is the principal carrier of fatty acids that are insoluble in circulatory plasma. BSA has a great affinity for fatty acids, hematin, bilirubin, etc. It can form covalent adducts with various metals such as Cu (II), Ni (II), Ag (II), Au (III) [12]. Kinetics of adsorption of serum albumins (human and bovine) to a hydrated metal oxide surfaces had been successfully investigated by Kurrat et al. [35,36]. Interactions of aqueous solutions of aluminum polyoxocations and aluminum hydroxide suspensions of varying particle sizes with BSA, have been investigated by Deschaume et al. [37]. Mikhaylova et al. [38] have introduced two different double-step immobilization approaches to study the immobilization of BSA on surface-modified superparamagnetic iron oxide nanoparticles. Fluorescence and absorption spectro- scopy are the most important technique to study the interactions of the metals with proteins because of its high sensitivity and relative ease of use. Interactions of different fluorophores with BSA have been studied by different authors [39–42]. In this paper we have exploited the fluorescence technique to study the interaction of BSA with gold nanoparticles of different sizes. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.06.008 à Corresponding author. Tel.: +9133 2414 6223; fax: +9133 24146584. E-mail addresses: sbjuchem@yahoo.com, scbhattacharyya@chemistry.jdvu.ac.in (S.C. Bhattacharya). Journal of Luminescence 128 (2008) 1969–1974