A cationic surfactant-decorated liquid crystal-based sensor for sensitive detection of quinoline yellow

Quinoline yellow (QY) is one of the popular synthetic food colorants and in food industry greatly used. Developing accurate and simple QY detection procedures is of major considerable importance in ensuring food safety. Hence, it is important to detect this food colorant effectively to reduce risk. Herein, an innovative liquid crystal (LC)-based sensor was designed for the label-free and ultra-sensitive detecting of the QY by means of a cationic surfactant-decorated LC interface. The nematic liquid crystal in touch with CTAB revealed a homeotropic alignment, when QY was injected into the LC-cell, the homeotropic alignment consequently altered to a planar one by electrostatic interactions between QY and CTAB. The designed LC-based sensor detected QY at the too much trace level as low as 0.5 fM with analogous selectivity. The suggested LC-based sensor is a rapid, convenient and simple procedure for label-free detection of QY in food industrial and safety control application.

www.nature.com/scientificreports/ the liquid crystal-aqueous interface causes homeotropic anchoring (through sidelong hydrophobic interaction between hydrocarbon chains of cationic surfactant and LC) 17 .
In the present research, 5CB (4-cyano-4′-pentylbiphenyl, a thermotropic nematic liquid crystal)-filled TEM copper grids were used by a cationic surfactant coverage, cetyltrimethylammonium bromide (CTAB) at the liquid crystal/aqueous interface. We expected that aqueous solution of QY would interact with CTAB, would disarrange the original orientation of liquid crystal and cause a QY detection through the homeotropic to planar orientational alert of the 5CB (Fig. 2). To examine this idea, we first studied interaction of CTAB and QY by   www.nature.com/scientificreports/ using absorption spectroscopy. Then we analyzed the limit of detection (LOD) and selectivity of this platform for QY detecting, based on the alteration in the optical pictures of liquid crystal. UV-Vis spectroscopy. The UV-Vis spectra were registered at ambient temperature on a SPEKOL 1500 UV-Vis spectrophotometer supplied with 1 cm quartz cells. The slit width was adjusted to 5 nm and the wavelength range was 200-500 nm. QY was dissolved in the DI water and diluted to 5.0 μM. In measuring of each data point, 50 μl of the CTAB solution (4.0 mM) was added to 2 ml of the QY solution and then UV absorbance spectra were measured.

Results and discussion
Investigation on the interaction between QY and CTAB. To investigate whether CTAB could interact with QY or not, spectrophotometric data were employed. Figure 3 shows the UV-Vis absorption spectra of QY in the presence of various concentrations of CTAB. The absorbance band intensity of QY at 484 nm decreased with increasing concentration of CTAB and the peak position shows a moderate blue shift, suggesting the formation of QY-CTAB complex. This result guide us to deduce that CTAB could willingly interact with QY to yield related complex 18 .

Optimization of CTAB concentration.
Previous studies have shown that to regularize the alignment of the liquid crystals at the LC-aqueous interface, CTAB could be used 19,20 . Adding of CTAB onto the DMOAPcoated glass imparted the homeotropic regulation of liquid crystals via the hydrophobic interactions between the   Fig. 4, entirely dark in appearance of liquid crystals were seen when the CTAB concentration ≥ 10 mM, corresponding to the homeotropic anchoring of the liquid crystals at the interface. Hence, a CTAB concentration of 10 mM was considered as the optimum CTAB concentration in following experiments.

Detection of QY by using LC-based sensor.
When an aqueous QY solution was inserted at the LC/ aqueous interface of liquid crystals, electrostatic interactions occurred between the anionic group of QY and cationic head groups of CTAB. The electrostatic pairing afforded the disruption of the self-assembly of CTAB at the interface and the primary homeotropic alignment of the liquid crystal molecules altered to a planar alignment. Following, we added QY solution into LC cell system to evaluate how the electrostatic interactions between CTAB and QY affect the orientations of LC and polarized optical microscope (POM) images 15 . Figure 5 indicates the polarized optical microscope pictures of the LC-cell in aqueous QY solutions at various QY concentrations. The bright regions in the images become greater with the concentrations of QY increasing from 0.05 to 5 × 10 5 fM, confirms that the binding of QY to CTAB can change the initial homeotropic orientation of the LC-cell to a planar orientation.
To quantitatively elucidation of the developed sensor efficiency, the mean grey value parameter of the POMs was achieved by means of ImageJ software (NIH Freeware). Figure 6 demonstrates the correspondence between www.nature.com/scientificreports/ the mean grey value and the logarithm of QY concentration. As the results show the detection limit of QY in this study is about 0.5 fM. Extraordinarily, the LC-based sensor quantitatively detected QY with a too much low detection limit. Compared to some available sensors for the QY detection, our assessment displays relative high sensitivity with a high performance as shown in Table 1.

Selectivity of LC-based sensor for QY detection.
The selectivity of the platform was assessed through basic red 46, basic violet 16, basic yellow 28, navy blue, dis E-3G dyes. The results in Fig. S1 shows that basic violet 16, basic yellow 28, navy blue and dis E-3G have no special binding to the QY sensor, therefore, the orientation of the liquid crystal molecules was conserved. Figure 7 shows the mean grey amounts of the various POMs. These outcomes confirm the selectivity of QY detection through designed liquid crystal-based sensor.

Conclusion
We reported a new and simple LC sensor based on a cationic surfactant-decorated LC interface for the detecting of QY, a hazardous food colorant. QY could disrupt the organization of the CTAB monolayer at the liquid crystal interface, therewith causing alter of the LC responses from dark-to-bright appearance. The optimum concentration for CTAB to design the sensor was 10 mM. The developed sensor has high selectivity for QY with lowest detection limit 0.5 fM, based on the alter in orientation and optical properties of the LCs for QY detection. This study introduces a simple, label-free and low-cost sensor for the detection of QY which displays promising and potential perspectives in sensing approaches.