Mesoporous halloysite nanotubes as nano-support system for cationic dyes: An equilibrium, kinetic and thermodynamic study for latent fingerprinting

Abstract

The absence of a cost-effective and less toxic latent fingerprinting powder, without compromising the quality of fingerprint developed, has always been felt by the crime scene investigators. In this regard, the present study aimed at using mesoporous halloysite nanotubes (Hal) with adsorbed cationic dyes viz. methylene blue (MB) and rhodamine 6G (R6G) to develop latent fingerprinting nano-powders. Batch adsorption studies showed that mesopores of Hal were able to adsorb more than 95% of both dyes at pH 7, when the reaction was carried out at 40 °C for 1 h, where concentration of dye and Hal was 250 μg/mL and 1 mg/mL, respectively. The equilibrium modelling revealed good mathematical fit in Langmuir isotherm and indicated towards monolayer adsorption for both dyes, while kinetic modelling showed that pseudo first order (PFO) was the most suitable kinetic model for both dyes. The reaction feasibility and chemisorption were confirmed through thermodynamic parameters (ΔG, ΔH and ΔS). Further, characterization techniques viz. SEM, TEM, EDX, ATR-FTIR, XRD, TGA and DSC proved the presence of MB and R6G in Hal. Furthermore, the latent fingerprinting nano-powders produced (MB@Hal and R6G@Hal) were used to develop fingerprints on different non-porous surfaces, where level 1 features of fingerprints were visible to the naked eye under daylight and 366 nm UV light. Level 2 (bifurcation, lake, ridge ending) and 3 (sweat pores) fingerprint features were easily seen under the microscope.