Spectroscopic investigation of two xanthane dyes and design of a FRET based pesticide sensor

Abstract

Layer-by-Layer (LbL) technique is the simplest and inexpensive method for preparartion of nano-dimensional thin films for tailoring material behavior having wide range of applications including sensors. Here, spectroscopic behavior of two laser dyes Acriflavine (Acf) and Rhodamine B (RhB) assembled onto LbL films have been investigated. It has been observed that both Acf and RhB form stable LbL films. Polyanion polyacrylic acid (PAA) was used to incorporate the Acf or RhB onto the LbL films. Adsorption of Acf and RhB onto PAA were completed within 45 min and 30 min respectively. During LbL film, material loss occurred in case of Acf. It has been demonstrated that such material loss can be minimized by incorporating clay laponite onto the LbL films. Temperature and pH dependant studies indicate that Acf and RhB assembled onto LbL films can be used to design temperature as well as pH sensors. Fluorescence Resonance Energy Transfer (FRET) between Acf and RhB has also been investigated. Interestingly, it has been demonstrated that the energy transfer efficiency can be manipulated using spacer molecules within the Acf and RhB LbL films. Laponite clay can be used to enhance the FRET efficiency, whereas stearic acid (SA) can be used to lower the efficiency. FRET efficiency linearly changes upon exposure of a pesticide pretilachlor at varying concentration. This study indicated that with proper calibration, proposed sensing system can be used to design FRET based pesticide sensor with detection limit of 0.22 ppm.

Keywords: Clay; Dyes; FRET; PAA; Pretilachlor; SA.

Fig. 1

Fluorescence spectra of pure Acf, pure RhB, Acf + RhB in presence of pretilachlor of different concentrations (0.25 ml/L, 0.5 ml/L, 0.75 ml/L, 1 ml/L). Inset shows the variation of FRET efficiency with pretilachlor concentration. Also the photographs of the Acf & RhB mixture in presence of pretilachlor at different concentrations are shown in the inset.

Fig. 2

(a) UV-Vis absorption and (b) Fluorescence spectra of Acf in solution, LbL film and microcrystal. (c) UV-Vis absorption and (d) Fluorescence spectra of RhB in solution, LbL film and microcrystal. Excitation wavelengths for fluorescence measurement, were 430 nm and 550 nm for Acf and RhB respectively.

Fig. 3

Plot of absorbance as a function of deposition time for (a) Acf and (b) RhB. Inset shows corresponding absorption spectra.

Fig. 4

Absorption spectra of (a) Acf and (b) RhB in LbL films with increasing layer number. Inset shows the plot of the absorbance as a function of layer number.

Fig. 5

Variation of absorption intensity of (a) Acf and (b) RhB with increasing and decreasing temperature. Variation of absorption intensity of (c) Acf and (d) RhB with increasing and decreasing pH.

Fig. 6

Plot of absorbance intensity as a function of deposited layers in PAA to study the material loss for (a) Acf (b) RhB (c) Acf in presence of clay.

Fig. 7

Fluorescence spectra of pure Acf, pure RhB and Acf + RhB mixture (50:50 volume ratio) in solution. Inset shows the excitation spectra of Acf and RhB at the emission wavelength monitored at 500 nm (Acf emission maximum) and 577 nm (RhB emission maximum).

Fig. 8

Fluorescence spectra of (Acf + RhB) in presence of PAA, (Acf + RhB) in presence of SA of 1 layer, (Acf + RhB) in presence of SA of 3 layers, and (Acf + RhB) in presence of clay.

Fig. 9

Schematic representations of FRET between Acf and RhB (a) without any spacer (b) with PAA as spacer (c) with clay as spacer (d) with 1 layer of SA as spacer (e) with 3 layers SA as spacer.

Fig. 10

Schematic of the closer association of Acf and RhB via hydrogen bonding with pretilachlor.

Fig. 11

FESEM images of (a) Acf + RhB (Concentrations are fixed at 10^− 4 M) and (b) Acf + RhB mixture after exposure to pretilachlor.