Sensing properties of silica nanoparticles functionalized with anion binding sites and sulforhodamine B as fluorogenic signalling unit

Silica nanoparticles functionalized with polyamidoamine (PAMAM) dendrimers as platforms for photoluminescence (PL) sensing of copper and cyanide ions

Functionalized nanoparticles for photoluminescence (PL) applications are a promising technology for biomedical imaging and as sensors for small molecules. This work presents a new method to modify silica nanoparticles (SNP) using the bifunctional linker 1,1'-carbonyldiimidazole (CDI) with a series of polyamidoamine (PAMAM) dendrimer molecules followed by grafting of fluorescein isothiocyanate (FITC) or rhodamine B isothiocyanate (RITC) to create platforms for photoluminescence (PL) sensors. A dendrimer size and charge-variable response to only copper(II) ions confirmed the prediction of a selective turn-off sensor via proximity quenching. Both dye density and Cu(2+) quenching efficiency peaked with SNP-dendrimer generation 4 (64 terminal amines). In addition, changing the terminal dendrimer arms to carboxylic acid end groups increased the copper quenching suggesting that more metal ion binding sites were created in close proximity to the dyes. Of the small anions tested for a turn-off sensor, only cyanide ion fully restored the PL when reaching a 2:1 CN(-):Cu(2+) ratio, while EDTA was not as effective at the same ratio. Therefore, dendrimer size and surface charge on the nanoparticles controlled the dye loading and copper quenching efficiency, while creating multiple binding sites for cyanide over other metal binding anions.

Fluoride ion sensing in aqueous medium by employing nitrobenzoxadiazole-postgrafted mesoporous silica nanoparticles (MCM-41)

A mesoporous silica-based inorganic-organic hybrid material (NBD-AP-MCM) has been designed and developed as a fluorescent chemosensor for the detection of fluoride in aqueous medium. The system was developed by covalently anchoring 7-nitro-2,1,3-benzoxadiazole (NBD) dye onto the surface of mesoporous silica nanoparticles, MCM-41. The system was characterized using several conventional analytical methods comprising spectroscopic, microscopic and thermo-gravimetric techniques. The sensory action of the material was investigated by carrying out steady state absorbance, fluorescence and time resolved fluorescence studies on the system in the absence and presence of several biologically and environmentally important anions in aqueous solution. The photophysical data of the present system (NBD-AP-MCM) have also been compared with the free dye (NBD) molecules. A significant decrease in the fluorescence quantum yield of the fluorophore in the hybrid material NBD-AP-MCM has been observed as compared to the unbound NBD. The decrease in fluorescence efficiency in the hybrid material is attributed to the aggregation caused quenching (ACQ) phenomenon. Interestingly, the system displays more than six-fold fluorescence enhancement in the presence of fluoride ions in aqueous solution. Enhancement of the fluorescence lifetime of the fluorescing moiety (NBD) has also been observed during fluorescence time-resolved studies. No significant optical changes have been observed with other commonly encountered anions rendering the present system highly selective towards fluoride detection. The fluorescence enhancement has been attributed to the cleavage of Si-O bonds due to the addition of fluoride. The silyl cleavage detaches the fluorophore from the solid support thereby making the fluorophore "free" in solution, which in turn recovers its original fluorescence which was decreased because of the aggregation on the solid silica support. Furthermore, the suitability of the present system in cellular imaging has also been demonstrated.