Published: November 03, 2011 r2011 American Chemical Society 9676 dx.doi.org/10.1021/ac202610e | Anal. Chem. 2011, 83, 9676–9680 ARTICLE pubs.acs.org/ac Copper Quantum Clusters in Protein Matrix: Potential Sensor of Pb 2+ Ion Nirmal Goswami, † Anupam Giri, † M. S. Bootharaju, ‡ Paulrajpillai Lourdu Xavier, ‡ Thalappil Pradeep,* ,‡ and Samir Kumar Pal* ,† † Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 098, India ‡ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 India b S Supporting Information T he drive toward optoelectronic nanodevices, 1 biosensors, 2 bioimaging, 3 nanoelectronics, 4 and novel catalysts 5 has gen- erated a need to synthesize new metal clusters. Compared to bulk materials, clusters are of fundamental interest due to their own intrinsic properties but also because of their intermediate posi- tion between molecular and materials science. Owing to their ultrasmall size, biocompatibility, and highly luminescent proper- ties, applications of these luminescent quantum clusters would be an attractive field to study. In the past few years, extensive studies have been performed on luminescent Au and Ag nanoclusters 6À10 using various templates such as peptides, 11 DNA, 12 thiols, 13 dendrimers, 14 polymers, 15 and proteins. 6 Among all of them, the use of a biomolecule as template or scaffold for synthesis possesses many advantages in biological applications. However, to date, only a few experimental studies have given direct insight into copper nanoclusters, 16À18 primarily because of the difficulty in preparing highly stable and extremely tiny Cu particles. Addi- tionally, subnanometer-sized Cu intrinsically suffers from un- stable colloidal dispersion of its particles and easy surface oxi- dation on exposure to air. Therefore, it would be of great interest to develop very stable, highly luminescent, biocompatible copper quantum clusters (Cu QCs) with emission in the visible range. Herein, we report the synthesis of Cu QCs by a simple one-pot chemical reduction method by using a commercially available protein, bovine serum albumin (BSA). It has been demonstrated that BSA can be used as the model protein for the synthesis and stabilization of gold nanoclusters. 6 The resulting Cu QCs were highly resistant to oxidation and exhibit photoluminescence and highly stable properties in a colloidal dispersion. The as-prepared blue-emitting clusters were assigned a molecular formula based on matrix-assisted laser desorption ionization mass spectrometry (MALDI MS). The as-synthesized quantum clusters were char- acterized thoroughly using various spectroscopic and micro- scopic techniques [UVÀvis, luminescence, transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray photo- electron spectroscopy (XPS), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC)]. The effect of oxidizing agent on the luminescence property of the cluster solution was probed. The luminescence of the QCs was exploited as a selective sensor for the detection of the toxic Pb 2+ ion. The reason of quenching was found to be aggregation manifested as revealed from our DLS study. To the best of our knowledge this is the first time protein-protected Cu QCs have been synthesized in a protein matrix and used as toxic metal ion sensor at part-per-million levels even in the presence of other interfering ions. Received: October 1, 2011 Accepted: November 3, 2011 ABSTRACT: A one-pot synthesis of extremely stable, water- soluble Cu quantum clusters (QCs) capped with a model pro- tein, bovine serum albumin (BSA), is reported. From matrix- assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, we assign the clusters to be composed of Cu 5 and Cu 13 cores. The QCs also show luminescence proper- ties having excitation and emission maxima at 325 and 410 nm, respectively, with a quantum yield of 0.15, which are found to be different from that of protein alone in similar experimental conditions. The quenching of luminescence of the protein- capped Cu QCs in the presence of very low hydrogen peroxide concentration (approximately nanomolar, or less than part-per-billion) reflects the efficacy of the QCs as a potential sensing material in biological environments. Moreover, as-prepared Cu QCs can detect highly toxic Pb 2+ ions in water, even at the part-per-million level, without suffering any interference from other metal ions.