Inhibition of Switch-on Fluorescence of Fluorochrome on Loading TiO2 with Gold

Gold loaded TiO 2 nanoparticles have been synthesized and characterized by powder XRD, HR-TEM, and EDX analysis. The binding interaction of uorescent sensor 5-amino-2-mercaptobenzimidazole (uorochrome) with TiO 2 and gold loaded TiO 2 nanoparticles has been discussed herein. The interaction of uorochrome with TiO 2 and gold loaded TiO 2 nanoparticles has been studied by UV-visible, uorescence, and FT-IR spectral techniques. The uorescence emission occurs at 421 nm and this has been selectively enhanced by TiO 2 nano semiconductor. This technique is sensitive to detect and estimate TiO 2 nano semiconductor at a micromolar level. This switch-on uorescence is suppressed when it is loaded with gold. The strong adsorption of uorochrome over the surface of nano semiconductor results in the electron transfer between uorochrome and nano semiconductor. Further, the binding site of nano semiconductor with uorochrome has been studied theoretically by using the molecular electrostatic potential (MEP). The results show higher electron density at the azomethine nitrogen atom.


Introduction
Among families of heterocyclic compounds, imidazoles play an important role because of its bioactivity and sensor properties. Imidazole and its derivatives have attracted increasing interest in the eld of chemical research because it acts as a precursor in synthetic reactions towards, primarily for the preparation of functionalized materials. Some of the well-known bio-components of human organisms such as Vitamin B 12 , amino acid histidine, DNA base structure components, histamine, purines, and biotin contain the imidazole nucleus as the main structure. Several synthetic drug molecules structure that contain the imidazole ring as the main component include cimetidine, azomycin, and metronidazole and also have signi cant application in the various elds [1,2].
Metal oxide nanoparticles have gained promising research attention due to its exclusive size-dependent optical and electronic properties. They are also applied in various biotechnological elds such as drug delivery, luminescence tagging, and immunoassay. As a result of recent advances in the eld, the interaction between organic molecules and the surfaces of the semiconductor materials is an interestingly booming area of research [3 − 6]. An organic molecule on surfaces of the semiconductor leads to the enhanced interaction of the semiconductor with an incident electromagnetic wave [7,8]. TiO 2 have been applied in the development of various technologies due to their excellent stability, low cost, non-toxicity, and physicochemical property. TiO 2 are generally used to improve their performance in many end-use applications. Reportedly, ethanol suspension of Au/TiO 2 , maintaining charge equilibrium by transferring photoexcited electrons from TiO 2 to Au nanoparticles [5]. Because of the usage of ethanol as a solvent, possible recombination of electrons in Au and holes in TiO 2 has not been studied [11][12][13][14][15].
Various studies have been developed for the quenching of the photoluminescence because of the charge from semiconductor nanoparticles [16][17][18][19][20]. The highest occupied molecular orbital and lowest unoccupied molecular orbital potentials for the designed sensor must match with the conduction and valence band edges of the semiconductor TiO 2 nanoparticles. In this contribution, we report uorescence enhancement by TiO 2 nanoparticles by 5-amino-2-mercaptobenzimidazole ( uorochrome). The observed uorescence enhancement is unique to study the interaction between virgin and Au loaded TiO 2 nanoparticles with uorochrome.

Reagents
Titanium (IV) isopropoxide was purchased from Sigma Aldrich. Tetrachloro auric acid was obtained from CDH chemicals. Milli-Q-water was used. All other reagents and solvents were used without further puri cation.

Preparation of Au loaded TiO 2 nanoparticles
Au loaded TiO 2 nanoparticles were prepared by a slight modi cation of the method described in the literature report [21]. Ti (IV) isopropoxide (20 mM) and acetylacetone (20 mM) in isopropanol (30 ml) was sonicated for 15 minutes. The sonicated solution was added to HAuCl 4 .3H 2 O (10 mM) mixed with milli-Q water (5 ml) and Dimethyl Furan (DMF) (20 ml) was added to it and stirred for 20 minutes. The mixture was then re uxed at 70 ºC for 2 h and the precipitate formed was sonicated for 2 h. To this mixture was added toluene and the colloidal material was precipitated, washed with toluene (3 × 10 ml), redissolved in isopropanol. The precipitate was kept at room temperature for 24 h to evaporate the solvent and nally, the Au loaded TiO 2 nanoparticles were isolated.

Instrumentation
XRD diffraction analysis was carried out using X'pert PRO PANalytical diffractometer operated at CuK α radiation (k = 1.5406 Å) source. JEOL JEM-3010 electron microscope was used to take TEM images with the magni cation of 600 and 800 k times operated at 300 keV. UV-visible analysis was obtained on Perkin Elmer Lambda 35 spectrophotometer. RXI spectrometer was used to obtain the FT-IR in the frequency range of 4000-500 cm − 1 using the KBr pellets.

Theoretical calculations
The classical point charge model was used to determine the molecular electrostatic potential (MEP). MEP of the molecule has been analyzed by moving a unit positive point charge across the van der Waals surface and calculated at various points ( j) on this surface using the relation as follows: Where, qipartial charge of each atom i and rji -distance between the points j and atom i  Fig. 2b. A closer look at the particles reveals that TiO 2 nanoparticles have been encapsulated with gold. The average particle size is nearly 48 nm. EDX spectrum (Fig. 3) shows that the successful loading of TiO 2 nanoparticles with Au.

Absorption characteristics of uorochrome -Au loaded TiO 2 nanoparticles
The absorption spectra of the uorochrome with and without loading of gold and unloaded TiO 2 nanoparticles are displayed in Fig. 4. The uorochrome absorbance was enhanced by the TiO 2 nanoparticles without the signi cant changes in the absorption maximum which indicates that nanoparticles do not alter the uorochrome excitation. The enhancement of absorbance is because of the adsorption of uorochrome on the TiO 2 surface. The enhanced absorbance by TiO 2 nanoparticles is suppressed by loading it with gold. uorescence by TiO 2 nanoparticles. This is due to the attachment of uorochrome more strongly to the unloaded TiO 2 nanoparticles than gold loaded TiO 2 nanoparticles.

FT-IR spectral studies
The nature of the interaction between the organic molecule and the semiconductor nanoparticles was further proved by the FT-IR technique. The FT-IR spectra of uorochrome, uorochrome functionalized TiO 2 nanoparticle, are displayed in Fig. 6.  Figure 7 shows the MEP map of uorochrome contains four basic sites, but in the binding process azomethine (> C = N-) nitrogen was involved due to the high electron density. Azomethine nitrogen having higher electron density which has been further proved by density functional theory (DFT) calculation which was used to obtain the MEP for the composites. The dark red region present in the molecular electrostatic potential map represents the most negative potential of the nitrogen atom. The green region predominance in the map represents the potential halfway (red and blue colour) between the two extremes [22].

Conclusion
A sensitive uorochrome uorescent sensor is adsorbed on the surface of Au loaded TiO 2 nanoparticles through active azomethine nitrogen. The position of conduction band energy determines the electron transfer from excited state uorochrome to Au loaded TiO 2 nanoparticles. The uorescent enhancement was demonstrated using a photo-induced electron transfer (PET) mechanism. Loading TiO 2 with Au hamper the sensitivity signi cantly reduces the same. The shift in absorption spectra was observed at 335 nm due to the adsorption of uorochrome on Au loaded TiO 2 nanoparticles surface. FT-IR results indicate the shift in > C = N-frequency at 1615 cm − 1 because of the binding of uorochrome on Au loaded TiO 2 nanoparticles. Azomethine nitrogen plays a major role in the binding process of the uorescent sensor with Au loaded TiO 2 nanoparticles which were con rmed by MEP studies.