RESEARCH PAPER Construction of a highly sensitive signal-on aptasensor based on gold nanoparticles/functionalized silica nanoparticles for selective detection of tryptophan Ayemeh Bagheri Hashkavayi 1 & Jahan Bakhsh Raoof 1 & Reza Ojani 1 Received: 27 May 2017 /Revised: 11 July 2017 /Accepted: 15 August 2017 # Springer-Verlag GmbH Germany 2017 Abstract In this work, a highly sensitive, low-cost, and label- free aptasensor based on signal-on mechanisms of response was developed by immobilizing the aptamer on gold nanopar- ticles (AuNPs)/amine-functionalized silica nanoparticle (FSN)/screen-printed electrode (SPE) surface for highly selec- tive electrochemical detection of tryptophan (Trp). The hemin (Hem), which interacted with the guanine bases of the aptamer, worked as a redox indicator to generate a readable electrochemical signal. The changes in the charge transfer resistance have been monitored using the voltammetry and electrochemical impedance spectroscopic (EIS) techniques. The peak current of Hem linearly increased with increasing concentration of Trp, in differential pulse voltammetry, from 0.06 to 250 nM with a detection limit of 0.026 nM. Also, the results obtained from EIS studies showed that the Trp was detected sensitively with the fabricated aptasensor in the range of 0.06–250 nM. The detection limit is 0.01 nM, much lower than that obtained by most of the reported electrochemical methods. The usage of aptamer as a recognition layer led to a sensor with high affinity for Trp, compared with control amino acids of tyrosine, histidine, arginine, lysine, valine, and methionine. The usability of the aptasensor was success- fully evaluated by the determination of Trp in a human blood serum sample. Thus, the sensor could provide a promising plan for the construction of aptasensors. Keywords Tryptophan . Hemin . Signal-on aptasensor . Functionalized silica nanoparticles . Gold nanoparticles Introduction Tryptophan (Trp) is a necessary amino acid which is required for normal growth, nitrogen equilibrium, and brain functions in humans [1]. It is also used as a precursor for melatonin [2], serotonin, and niacin [3]. Trp cannot be synthesized in the human body and therefore must be achieved from food or supplements. Trp, rarely found in vegetable products, thus sometimes added to food products, dietary, and pharmaceuti- cal formulations [4]. Regrettably, when it was incorrectly me- tabolized, it begets a toxic product in the brain that causes schizophrenia, hallucinations, and delusions [5]. The concentration of free Trp in blood plasma in patients such as depressive patients, liver cancer patients, colorectal cancer patients, and autistic patients is considerably different than the normal control [6–8]. On the other hand, because the concentration of Trp in biological samples is low, therefore, there is an urgent need for rapid, easy, sensitive, selective, and less expensive techniques for quantification of Trp. Many methods such as spectroscopy (spectrophotometry) [9], capillary electrophoresis [10], chemiluminescence [11], and high performance liquid chromatography [12] have been applied for the determination of Trp. Although these methods are able to identify Trp, but they are often complex, tiresome, time-consuming, expensive, non-selective, and require trained personnel. Thus, it is essential to look for a particular sub- stance that can specially connect to analytes. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00216-017-0588-z) contains supplementary material, which is available to authorized users. * Jahan Bakhsh Raoof j.raoof@umz.ac.ir 1 Electroanalytical Chemistry Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar 47416-1467, Iran Anal Bioanal Chem DOI 10.1007/s00216-017-0588-z