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Surface functionalization of teeth with fluoride-incorporated apatite layers displays great potential in treatments and prevention of dental disorders. In this study, we used a sintered hydroxyapatite (sHA) substrate as a model material of teeth, and established a rapid and area-specific coating technique of fluoride-incorporated apatite layers by using a laser-assisted biomimetic (LAB) process. In this technique, a sHA substrate was irradiated on the surface with a Nd:YAG pulsed UV laser for 30 min in supersaturated calcium phosphate (CaP) solutions with various fluoride concentrations. The fluoride concentration in the CaP solution was varied to control morphology, crystalline structure, and fluoride content of the resulting layers. Without fluoride in the CaP solution, an octacalcium phosphate (OCP) layer with a flake-like structure was formed on the laser-irradiated surface of the substrate. The addition of fluoride (1000 µM and 3000 µM) to the CaP solution led to the formation of fluoride-incorporated apatite layers with an enamel-like needle-like nanostructure. The fluoride-incorporated apatite layers adhered firmly to the sHA surface and reduced acid dissolution of the sHA substrate by acting as a protective covering. Additionally, the layers released fluoride ions for more than 24 h, and exhibited antibacterial activity relative to a caries-causing bacterium, namely Streptococcus mutans. Thus, our LAB process can potentially act as a new tool for functionalization of tooth surfaces. Statement of Significance We used a sintered hydroxyapatite (sHA) substrate as a model material of teeth, and established a rapid and area-specific coating technique of fluoride-incorporated apatite layers on the sHA surface by using our laser-assisted biomimetic (LAB) process. In this process, pulsed laser was utilized to accelerate seeded crystal growth in supersaturated calcium phosphate solutions supplemented with NaF. The thus-fabricated fluoride-incorporated apatite layers consisted of enamel-like needle-like nanocrystals with c-axis orientation. These fluoride-incorporated apatite layers adhered firmly to the sHA surface, reduced acid dissolution of the sHA substrate by acting as a protective covering, and exhibited antibacterial activity against Streptococcus mutans through the fluoride release. Thus, our LAB process can potentially act as a new tool for functionalization of tooth surfaces.

ZnO nanorods (NRs) of diameter ~50 nm are coated with Ag@SiO2 NPs by the two-step chemical method. The structural and morphological properties of the Ag@SiO2 NPs coated ZnO NRs confirm its structural purity as well as adhesion of Ag@SiO2 NPs onto the NRs׳ surface where thickness of the SiO2 coating has been changed. The photoluminescence (PL) properties of the coated NRs show that the intensity of the near band edge emission peak has been enhanced to as high as 7 times as compared to only ZnO NRs when a ~12 nm thick SiO2 shell exists around Ag core which acts as a spacer layer in between Ag and ZnO. The mechanism behind the enhancement is has been successfully explained by interpreting the PL results with a model which would further help one to understand similar PL enhancement in similar metal-semiconductor systems.

ZnO nanorods (NRs) have been decorated by Au nanoparticles (NPs) by a chemical method. The ultraviolet (UV) photoresponse of Au/ZnO NRs has been investigated. As the loading of Au NPs increases, the photocurrent as well as the photo-to-dark current ratio (gain) increases attaining a maximum gain value which is ∼15 times higher than that of the pristine ZnO NRs. Photoresponse enhancement is probably due to efficient separation of photo-generated electron-holes by an enhanced electric field and hot carrier injection over the Au localized Schottky junctions.

Cu/ZnO nanorods (NRs) hybrids have been synthesized for various concentrations of Cu nanoparticles (NPs). X-ray diffraction patterns and X-ray photoelectron spectroscopy show the phase purity of the hybrids. A detailed study on the photoluminescence (PL) properties of Cu/ZnO NRs hybrids show that the near band edge emission (NBE) of ZnO is enhanced eight times after Cu loading over that of pristine ZnO NRs. The mechanism behind luminescence enhancement has been analyzed by the surface plasmon dispersion relations of Cu/air and Cu/ZnO systems. It has been confirmed that the enhanced emission is due to the surface plasmon resonance coupling of the NBE emission between Cu and ZnO.

A detailed study on the emission properties of ZnO nanorods/Ag nanoparticles for various concentrations of Ag has been presented. The photoluminescence (PL) spectra show that the near band edge (NBE) luminescence of ZnO is enhanced 10 times after Ag loading leading to an ultraviolet/visible emission ratio to 12.2 compared to a value of 1.2 for pristine ZnO nanorods. The enhancement in PL intensity is due to surface plasmon resonance coupling of the NBE emission between Ag and ZnO. We further confirm that morphology and/or surface modification do not contribute to this large increase in the band edge emission.

Ag@SiO2/ZnO nanorods (NRs) heterostructures have been grown by chemical solution method in two steps. The band edge ultraviolet (UV) emission intensity of ZnO NRs has been enhanced four times as Ag@SiO2 nanoparticles are loaded on NRs' surface. The emission enhancement is due to surface plasmon resonance of the metal NPs facilitated by the SiO2 shell around it.

We recently developed a rapid single-step calcium phosphate (CaP) precipitation technique on several substrates using a laser-assisted biomimetic process (LAB process). In this process, ultraviolet (UV, λ = 355 nm) pulsed laser irradiation has been applied to a substrate that is immersed in a supersaturated CaP solution. In the present study, the LAB process for CaP precipitation on a titanium substrate was successfully expanded to include not only UV but also visible (VIS, λ = 532 nm) and near infrared (NIR, λ = 1064 nm) lasers. Surface heating and plasma-mediated surface reactions (micro-deformation, oxidization, photoexcitation, and wetting) generated by UV, VIS, or NIR lasers are considered to be involved in the CaP precipitation on the titanium surface in the LAB process. The kinetics of these reactions and consequently of CaP precipitation were dependent on the laser wavelength and fluence. The higher laser fluence did not always accelerate CaP precipitation on the substrate; rather, it was found that an optimal range of fluence exists for each laser wavelength. These results suggest that for efficient CaP precipitation, a suitable laser wavelength should be selected according to the optical absorption properties of the substrate material and the laser fluence should also be adjusted to induce surface heating and plasma-mediated surface reactions that are favorable for CaP precipitation.

Ag-ZnO nanorods (NRs) have been grown by solution method by capping aqueous chemically grown ZnO NRs by solutions of Ag nanoparticles (NPs) with different concentrations. The band edge ultraviolet (UV) emission intensity of ZnO increases gradually as the Ag concentration increases and becomes 3.2 times higher than the pristine NRs. Also a 3.2 times enhanced UV photocurrent has been obtained in Ag-ZnO NRs.

Information on the mechanistic differences in the luminescence properties of Ti/ZnO nanorods (NRs) has been obtained through the preparation of heterostructures by (a) varying the thickness of Ti from 1 nm to 20 nm keeping the substrate temperature at 400 °C, (b) varying the substrate temperature from room temperature (RT) to 500 °C while keeping the metal thickness constant at 10 nm and (c) annealing the RT Ti sputtered NRs at temperatures of 400 °C and 500 °C. The photoluminescence (PL) spectra show that the near band edge luminescence of ZnO in the ultraviolet (UV) region is enhanced as the thickness of Ti increases up to 5 nm and, thereafter, it falls. Sputtering of Ti on ZnO NRs at RT does not cause any UV enhancement but when sputtered at and above 400 °C, the UV intensity is enhanced. Annealing of RT Ti sputtered NRs at and above 400 °C also results in the enhancement of the UV peak, although with a lesser magnitude. Analysis of the PL results, supported by X-ray diffraction, field emission scanning electron microscopy, elemental mapping, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy and electrical I–V measurement results, show a clear indication that the surface diffusion of Ti causes a reduction in the surface defects.

SiO2 spacer layers of various thicknesses in between an Au and ZnO nanorod (NR) layer (Au/ZnO) have been introduced and the luminescence properties of the layered ensemble have been investigated. When ZnO NRs are grown directly on a granular Au layer, the ultraviolet (UV) emission of ZnO was enhanced nearly 2 times due to surface plasmon resonance (SPR) mediated charge transfer between Au and ZnO. A significant improvement is observed when the SiO2 spacer layer is introduced in between Au and ZnO. The UV/VIS emission intensity ratio of Au/SiO2/ZnO NRs' becomes ∼6 times higher than that of the pristine ZnO NRs. A SPR mediated electron transfer from Au/SiO2 to ZnO causes such an enhanced UV emission. The spacer diminishes the reverse electron transfer process from the conduction band of ZnO to the Fermi level of Au, a most competitive reason for PL quenching, which also accounts for the emission enhancement.