Cadmium-based QDs are one of the most widely used semiconducting QDs, as they have useful properties for biochemical sensors, biomedical imaging, photovoltaic applications, light-emitting diodes (LEDs), laser, thin-film transistors, or solar cells. A clear negative consequence of this extensive use is the discharge into the environment of such noxious material. To prevent their uncontrolled discharge, it is essential to develop simple methods to detect them in waste or even in the environment. The implementation of paper sensors for color analysis provides fast response, simple operation, and low cost in the detection of CdSe QDs.

Here we report the immobilization on cellulose paper of a previously reported [1] chemosensor for CdSe‑Cys QDs. The cellulose paper was first primed with 3-aminopropyl)trimethoxysilane (APTMS) by soaking it in a DMF solution for 2 h. In a second reaction step, N-hydroxysuccinimide (NHS) and N-(3-dimethylaminopropyl)-N´-ethylcarbodiimide hydrochloride (EDC·HCl) were added together with the chemosensor to achieve its immobilization on the amine-modified paper (by forming an amide bond) [2].

The observation of three dominant signals located around 270, 330 and 420 nm in the diffuse reflectance spectrum of the chemosensor-modified paper demonstrated its immobilization. The absorption bands appear around 50-80 nm red-shifted, with respect to the spectrum of an ethanol solution of the chemosensor (220, 250 and 340 nm).

The chemosensor-modified cellulose paper upon soaking in CdSe-Cys QDs water solutions for 2 h exhibits a decrease in the absorbance of each band of the diffuse reflectance spectrum. As the decrease is more pronounced in the band observed at 270 nm, we have chosen this wavelength to carry out the measurements.

To study the interaction between CdSe-Cys QDs and the chemosensor (H2SB), we obtained Cd2(SB)2(H2O)4 in solution and compared its UV-Vis spectrum with that of the reaction product between H2SB and CdSe-Cys QDs. The similarities between these spectra evidenced an interaction via metal-ligand coordination between CdSe-Cys QDs and H2SB.