Direct detection of cysteine using functionalized BaTiO₃ nanoparticles film based self-powered biosensor

Highlights

• First report on facile and direct detection of cysteine through I-V technique.

• Functionalized BaTiO₃ NPs for self-powered cysteine biosensing system.

• Biocompatible BaTiO₃ NPs and agarose biopolymer paves way for green chemistry.

• Proposed sensor has good selectivity and detection limits down to 10 µM (3 s/m).

• The findings may further lead to novel piezoelectric-biosensing devices.

Abstract

Simple, novel, and direct detection of clinically important biomolecules have continuous demand among scientific community as well as in market. Here, we report the first direct detection and facile fabrication of a cysteine-responsive, film-based, self-powered device. NH₂ functionalized BaTiO₃ nanoparticles (BT-NH₂ NPs) suspended in a three-dimensional matrix of an agarose (Ag) film, were used for cysteine detection. BaTiO₃ nanoparticles (BT NPs) semiconducting as well as piezoelectric properties were harnessed in this study. The changes in surface charge properties of the film with respect to cysteine concentrations were determined using a current–voltage (I-V) technique. The current response increased with cysteine concentration (linear concentration range=10 µM–1mM). Based on the properties of the composite (BT/Ag), we created a self-powered cysteine sensor in which the output voltage from a piezoelectric nanogenerator was used to drive the sensor. The potential drop across the sensor was measured as a function of cysteine concentrations. Real-time analysis of sensor performance was carried out on urine samples by non-invasive method. This novel sensor demonstrated good selectivity, linear concentration range and detection limit of 10 µM; acceptable for routine analysis.

Introduction

Cysteine, an essential amino acid containing a thiol group, has vital roles in homeostasis and as a precursor; it has also been used as a biomarker (Jung et al., 2012a). Deviations in cysteine concentration from physiological levels (30–200 µM) has been linked to chronic diseases, such as rheumatoid arthritis, Parkinson's disease, Alzheimer's disease, and even adverse pregnancy outcomes (Jung et al., 2012b; Yang et al., 2016; Gong et al., 2015; Zhang et al., 2007). Thus, a facile, direct method for cysteine detection could have considerable significance, given the currently available approaches, such as those based on high-performance liquid chromatography (Nekrassova et al., 2003), colourimetric and fluorescence spectroscopy (Jung et al., 2012b), capillary electrophoresis, and electrochemical voltammetry (Park et al., 2014). The existing methods have certain drawbacks such as complex sample preparation before measurement, time-consuming processes, and the need for sophisticated instrumentation. With this in mind, there is a continuing need for the development of a simple and rapid sensor capable of identifying cysteine in routine analysis.

One possible approach is a stimulus-responsive nanodevice; these devices have gained much attention among researchers due to their vital applications in theranostics, biomolecule detection (Sun et al., 2014), and drug delivery (Xia et al., 2007), avoiding off-target effects and false-positive results. A promising route to a stimulus-responsive system involves nanocomposites, comprising nanoparticles and polymers. Nanocomposites have been used in diverse applications in bio-sensing, electronics, drug delivery, nano-medicine, and catalysis (Thoniyot et al., 2015) due to their enhanced functionality. Typically, the polymer plays the role of a matrix (substrate) and functionalized nanoparticles (NPs) are involved in sensing the target molecule. In this study, an agarose (Ag) biopolymer was used as the substrate and amine-functionalised BaTiO₃ NPs (BT-NH₂ NPs) were used for the selective detection of cysteine for the first time. Agarose, a polysaccharide consisting of alternating residues of α-1,3-linked D-galactose and R-1,4-linked 3,6-anhydro-R-L-galactopyranose, forms left-handed helices as a linear polymer. These helices aggregate into long fibres, which, in turn, associate in three dimensions to form a three-dimensional (3D) gel network (Horger et al., 2009). Agarose has considerable chemical, thermal, and physical stability and is less likely to interact with biomolecules, making it a preferred matrix for use with protein and nucleic acids. Agarose films have higher stiffness than poly(vinylidene fluoride) films due to inter- and intra-hydrogen bonding of its functional groups. Also, the unique chemical functionality of agarose has prompted its use as an alternate electrode material and multifunctional green battery material (Hwang et al., 2016).

The BaTiO₃ (BT) nanoparticles used here are a well-known metal oxide with an ABO₃ perovskite structure, having (n-type) semiconducting (Selvarajan et al., 2016) and inherent piezoelectric properties (Park et al., 2012). Its unique features, such as biocompatibility, second-harmonic generation (SHG) (Hsieh et al., 2010), low dielectric constant, and high piezoelectric coefficient, make it a potential candidate for applications in biosensing (Selvarajan et al., 2016), theranostics (FarrokhTakin et al., 2013), bioimaging (Ĉulić-Viskota et al., 2012), and piezoelectric-based energy harvesting (Alluri et al., 2015). Furthermore, such ceramic perovskite-structured metal oxides have applications in electrical and electronic devices. As yet, the fact that this has still not been exploited extensively for biological applications renders great interest for self-powered biosensing and theranostic applications. Self-powered systems with novel features, such as battery-less operation, portability, point-of-care diagnosis, and implantable applications (Yu et al., 2013), can be developed using piezoelectric nanogenerators (PNG), a relatively old energy-harvesting technology in which mechanical energy is converted into electrical energy. Self-powered nanosensors combine the nanogenerator with a sensor, through internal or external integration. Thus, the energy harvested from the nanogenerator is used to drive the sensor. Based on the novel approaches mentioned above, in the present work, a self-powered cysteine sensor was demonstrated by externally integrating a cysteine sensor with a PNG. Ag/BT-NH₂ film-based cysteine sensor with metal–semiconductor–metal (MSM) configuration was connected in parallel to a BT/Ag film-based piezoelectric nanogenerator (BT/Ag PNG). The cysteine sensor's analytical outputs were analysed using a current–voltage (I–V) technique. In a self-powered cysteine sensor, the voltage across the sensor decreases with an increase in the cysteine concentration; this potential drop is measured as the sensing signal, in accordance with I–V studies. To our knowledge, this is the first report of a self-powered cysteine sensor based on a direct detection technique and BT NPs. The as-fabricated sensor demonstrated comparable or enhanced performance, compared with conventional sensors. The novelty in our work stems from the direct, self-powered detection method using functionalized BT NPs, harnessing both its semiconducting and piezoelectric properties. The proposed sensor can be developed as a possible prototype for implantable and point-of-care diagnostic devices in the near future.