Contents lists available at ScienceDirect Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Monitoring the binding of serotonin to silver nanoparticles: A fluorescence spectroscopic investigation Bikash Chandra Swain a , Padmaja Prasad Mishra b , Hirdyesh Mishra c , Umakanta Tripathy a, ⁎ a Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004, Jharkhand, India b Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata, 700064, West Bengal, India c Department of Physics, MMV, Banaras Hindu University, Varanasi, 221005, India ARTICLE INFO Keywords: Serotonin Silver nanoparticles (AgNPs) Fluorescence spectroscopy Quenching Neurotransmitters ABSTRACT Serotonin (5-HT) is an important monoamine neurotransmitter that plays a vital role in the regulation of various cognitive and behavioural functions including sleep, mood, pain, depression, anxiety, aggression, learning etc. From the nanotoxicity and neurotoxicity point of view, interaction studies between serotonin and nanoparticles are highly essential to develop an indispensable understanding on the effect of nanoparticles on monoamine neurotransmitters. In the present work, steady-state and time-domain fluorescence measurements were carried out along with binding energy measurements through X-ray photoelectron spectroscopic (XPS) technique to understand the interaction of 5-HT with silver nanoparticles (AgNPs). The emergence of a week red absorption band and quenching of fluorescence intensity along with decrease in full width half maximum, but not the excited state decay time of 5-HT in the addition of AgNPs suggest the formation of a non-fluorescent complex in the ground state that indicates static quenching along with radiative energy transfer among them. This is also confirmed by the low energy shift of Ag transition in 3 d 3/2 and 3 d 5/2 in XPS measurements and a coupling of L b transition with the surface plasmons of AgNPs in the excitation spectra. The obtained thermodynamic para- meters from the fluorescence quenching reveal the nature of interaction between 5-HT and AgNPs is hydro- phobic. The increase in binding constant with increasing temperature suggested an enhancement in the strength of this interaction due to rise in the mobility of the molecules and hence increases the sensitivity and association of 5-HT with AgNPs. 1. Introduction The widely studied metal-nanoparticles represent an admirable biocompatibility and hence, the combination of nanoparticles with biological relevant molecules is a rapidly growing research field at the crossroads of materials science, nanoscience, and molecular bio- technology [1–4]. The unique chemical and physical properties of such hybrid materials lead to the invention of nanoelectronic devices [5,6], and allow promising applications in other fields such as biosensors [7], biomedical including diagnostic & therapeutic treatment of diseases [8,9], detection of pathogenic agents [10,11], drug delivery etc. [12–15]. This is already having an impact in bioanalysis, where nano- particles of various sizes, shapes, and compositions are being used to replace “traditional” bioanalysis schemes. Additionally, the localization or propagation of the so-called surface plasmon resonance (SPR) of the nanoparticles also strongly depends upon the dimensionality of the nanostructured materials. The collective oscillation of conduction electrons at the metal surface regulates the confinement of light, which is responsible for the peculiar optoelectronic properties of such nano- materials. Their absorption band can be tuned by changing the size and shape of nanoparticles. Metallic nanoparticles like silver, gold and copper show localized surface plasmon resonance property and also used in fluorescence enhancement [16,17]. As the damping processes of nanoparticle plasmons have contribution of radiative and non-radiative processes, any excited fluorophore during interaction can transfer en- ergy to nearby metal nanoparticles to create surface plasmon modes. If the excited fluorophore is coupled to the dipole mode of the nano- particle, the plasmons decay like both radiatively and non-radiatively; if it is coupled to the high-order modes, they dissipate energy only non- radiatively. Many theoretical and experimental studies have been re- ported on the energy transfer rate from a fluorophore to a metal na- noparticle leading to the fluorescence quenching [18,19]. There is equally an increased concern about the impact of nano- particles (NPs) in the environment due to their inevitable release and https://doi.org/10.1016/j.jphotochem.2018.08.041 Received 10 July 2018; Received in revised form 25 August 2018; Accepted 26 August 2018 ⁎ Corresponding author. E-mail address: utripathy@iitism.ac.in (U. Tripathy). Journal of Photochemistry & Photobiology A: Chemistry 367 (2018) 219–225 Available online 27 August 2018 1010-6030/ © 2018 Elsevier B.V. All rights reserved. T