br Conflict of interest br Introduction Traditional labels s

Conflict of interest
Introduction Traditional labels such as fluorophores, chromophores, or radioactive labels are widely used in microfluidic applications to visualize flow [1] or detect the presence or concentration of relevant species [2,3]. However, each of these labels has shortcomings including bleaching of fluorophores, non-specificity of chromophores, and steric blocking of conjugated labels [4–6]. In particular in microfluidics, labeling with multiple dyes may suffer from the low Reynolds number laminar flow and when introduced at high concentration, may alter the physical properties of the flow solution. Label-free detection based on optical techniques has revolutionized the ability to detect a broad range of biological samples such as protein–protein interaction, AMG 925 simply based on the intrinsic dielectric permittivity of samples without the need for labeling [7–9]. In particular, label-free optical techniques such as surface plasmon resonance [10], photonic crystal [11], and ring resonator [12] have been integrated with microfluidics to measure the changes in the refractive index near the surface of the sensor. However, these techniques require complex instrumentation for illumination and detection such as high-resolution spectrophotometer, high-intensity monochromatic light source, and prism coupling. In contrast, optical sensors consisted of nanohole array on a noble-metal film, which exhibit extraordinary optical transmission (EOT) [13], can be used with similar sensitivity to detect the changes in the refractive index near the surface of the sensor, with the advantage of simple collinear broadband illumination and portable spectrophotometer [14–16]. Previously, we have reported colorimetric surface plasmon resonance imaging using a nanohole array device called nanoLycurgus Cup Array (nanoLCA), in which we demonstrated high spectral sensitivity of the nanoLCA to the changes in the refractive index due to the presence of different refractive index solutions and to the surface binding of biological-relevant molecules such as protein–protein interaction and DNA hybridization [17]. Due to the unique transmission/reflection peak wavelengths of nanoLCA in the visible wavelength range, we were able to demonstrate visible colorimetric changes simply due to the changes in the refractive index on and near the surface of the sensor upon the changes of the presence or the concentration of the sample of interest. In this work, we extended the capability of nanoLCA to on-chip applications by integrating relevant microfluidic designs, such as parallel flow channels and droplet generator, and by demonstrating colorimetric visualization of static and transient dynamics of optically-transparent solutions, which previously could not be visualized in a colorimetric manner.
Materials and methods
Results and discussion
Conclusion
Conflict of interest
Introduction Fast, sensitive and selective detection of 2,4,6-trinitrotoluene (TNT), an explosive, is imperative because of national security and environmental health concerns. The industries, military and terrorists have often used TNT as one of the explosive materials [1]. Because of the excessive use of TNT in war and structure demolition, it is contaminating the soil and groundwater, which causes adverse effects for human health [2–6]. It is well-known that TNT is toxic and carcinogenic and causes anemia, abnormal liver function, and cataracts [1–7]. Recently, numerous techniques have been developed for the detection of TNT, such as colorimetry [8–14], electrochemical approaches [15–18], luminescence [15,19–36], Raman spectroscopy [17,19,26,37–39], analytical paper devises [8,10,12,14,17,40], and others [33–36,41–44]. Most of these techniques exhibit good sensitivity and selectivity. Some require complicated instrumentation, but others use simple paper-based sensors [8,10,12,14,17,40]. Several reports took advantage of the Meisenheimer complex formation between TNT and an amine [11,15,32,38,45–47]. Here we report the binding study using absorption spectroscopy and NMR between 3-aminopropyltriethoxysilane (APTES) and TNT, as well as three TNT-like molecules, 2,4,6-trinitrophenol (TNP), 2,6-dinitrotoluene (DNT), and 4-nitrophenol (NP), to understand the selective APTES–TNT complex formation over other nitroaromatics. We further used an APTES-coated filter paper for selective detection of TNT.