Surfactant chain length controls photoinduced electron transfer in surfactant bilayer protected carbon nanoparticles Somen Mondal a , Tarasankar Das a , Prasun Ghosh a , Arnab Maity a , Arabinda Mallick b , Pradipta Purkayastha a,n a Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India b Department of Chemistry, Kashipur MM Mahavidyalaya, Purulia 723132, India article info Article history: Received 15 October 2014 Accepted 20 November 2014 Available online 29 November 2014 Keywords: Carbon materials Nanoparticles Surfactant double layer Photoinduced electron transfer abstract Negatively charged fluorescent carbon nanoparticles (CNPs) were synthesized from amino acids with bilayer of cationic surfactants of varying chain lengths on the surface. Degree of photoinduced electron transfer (PET) between an electron donor in the bilayer and the CNP core can be efficiently controlled depending on the thickness of the bilayer. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Photostable and optically bright metal nanoparticles and nano- clusters are good substitutes for biologically toxic semiconductor quantum dots (QDs) [1,2]. Carbon nanoparticles (CNPs) or nano- dots significantly substitute QDs. They can be surface passivated by organic compounds or biomolecules to acquire strong fluorescence and are physicochemically stable [3]. Recently we have reported synthesis and application of surfactant double layer coated fluor- escent CNPs toward fluorescence resonance energy transfer [4]. Here we report application of CNPs coated with bilayers of cationic surfactants, namely, cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB) and dodecyltri- methylammonium bromide (DTAB), in controlling photoinduced electron transfer (PET) between an electron donor and the core of the CNP aggregate. PET is an important phenomenon in many chemical and biological processes [5–7]. Occurrence of PET from a typical electron donor, dimethylaniline (DMA), to and from carbon dots has been studied in homogeneous and heterogeneous media [7]. Herein, we have shown that the extent of PET between DMA and CNPs can be effectively controlled by varying the thickness of the surfactant shell. 2. Materials and methods Fluorescent CNPs were prepared from cystine by microwave treatment following a reported protocol [8]. Briefly, 1 g of cystine was dissolved in 10 ml aqueous solution of sodium hydroxide and sonicated until a clear solution was obtained which was put into a microwave oven at 150 1C and incubated for 30 s. The resulting yellow solution was centrifuged at 25,000 rpm for 20 min and the supernatant containing CNPs was collected and preserved at 4 1C. Experimental concentration of CNPs was maintained at 0.45 mg/mL. The synthesized CNPs were characterized spectroscopically (Fig. S1). Atomic force microscopy (AFM) and dynamic light scattering (DLS) were used to determine their size (  25 nm) (Fig. S2) [4]. Strong green fluorescence, independent of excitation between 350 and 430 nm, is observed with a maximum at 506 nm. The luminescence of the CNPs decays multi-exponentially with an average fluorescence lifetime of 4.16 ns (at 506 nm) on exciting at 402 nm. The core–shell CNPs were produced by self-assembling of the cationic surfactants, CTAB, TTAB or DTAB, on CNP surface in aqueous medium. Aggregation of CNPs due to the surfactant shell increases the particle size (Fig. S2). Change in zeta potential of the CNPs from negative to positive as a function of surfactant concentration proves formation of double layer. 3. Results and discussion Absorbance of CNPs increases with addition of surfactants (Fig. S3A–C). Fluorescence from CNPs intensifies with increase in concentration of the surfactants along with a hypsochromic shift Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.11.104 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: ppurkayastha@gmail.com (P. Purkayastha). Materials Letters 141 (2015) 252–254