Nanomaterials as sensor are given a prime focus as it has enhanced the responsiveness, selectivity, reproducibility and detection limit of the sensors [1] [2] . Among them, 1D and 2D semiconducting nanostructures like nanorods, nanowires etc are particularly attractive as sensors as it has high surface-to-volume ratio, which leads to higher sensitivity [3] [4] . Divalent heavy metal ions play an important role in the biological activities such as O₂ movement, carbon fixation, methylation, phosphorylation, electron transfer mechanism as well as in the stabilization of bio molecules [5] . Also it is evident that uncontrolled human practice has modified the natural cycle of heavy divalent metals including the toxic non-essential elements like Pb, Hg etc. These modifications have become a serious concern for global sustainability. Consequently many efforts have been carried out by the research community to develop different kinds of sensors for monitoring and detecting heavy metal ions in food, drinking water, and other biophysical system in nature [6] -[8] .

Till today, many kinds of novel 1D nanostructured materials have been successfully synthesized, such as III - V [9] , II - VI semiconductors [10] [11] , elemental and oxide nanowires/Nanorods [12] [13] . To fabricate nanowires of various material, diverse experimental procedure have been reported by a variety of nanofabrication techniques [14] [15] and crystal growth methods. Among these techniques, arc discharge [16] , laser ablation [17] and catalytic CVD [18] growth are few prominent ones. Apart from these, the other most important methods of preparation of 1D structures are the catalyst- and template-based methods in which the catalysts provide the energetically favoured sites for the adsorption of gas reactants, while the templates/substrates are used to control and direct the growth of the 1D structure. The size and shape of the formed 1D structures therefore depends greatly on the selection of suitable catalysts as well as templates, which apparently involves complications [19] . The anodic alumina membrane (AAM) is among the several templates used generally for growing 1D structures; it is hard with uniform, nearly parallel and ordered pore structure [20] . Among various groups, the growth of MnS rods on the AAO template using Al (99.999%) via a two-step anodization [21] , growth of metal telluride/carbon nanocables via Te/carbon nanocables [22] , the fabrication of a coaxial silica shell on the surface of CdTe nanowires via the base-catalyzed hydrolysis of tetraethyl orthosilicate [23] are few examples of template assisted 1D structure growth.

In this article we demonstrate a simple and convenient method for the growth of luminescent CdTe nanorods on the anodized aluminum oxide (AAO) template by the hydrothermal method at a temperature of 180˚C for 4 hours without any catalyst. Also the hydrothermal method in the present case is milder and less harmful to the environment because of the use of stable sodium tellurite (Na₂TeO₃) as the tellurite source. Thus obtained CdTe nanorods on the AAO templates were used as a fluorescence probe for the detection of divalent ions such as Pb, Hg, which has adverse effects on human health, environment and for some essential microelements such as Zn and Mg.

All analytical grade chemicals were purchased from sigma Aldrich and were utilized without further purification. The AAO templates were purchased from Whatman. The stock solution of Hg²⁺, Pb²⁺, Mg²⁺, Zn²⁺ ions were prepared by dissolving suitable amount of compounds like HgCl₂, MgCl₂, Pb (CH₃COO)₂, and ZnCl₂, in deionized water. The molarities of the solution prepared were maintained at 1µM concentration.

The experimental procedure of CdTe nanorods on AAO template is depicted in Figure 1. In a typical synthesis, the commercially available AAO template having anodic circles of diameter 25 mm with pore size of 0.1 µm was first treated with organic solvents such as acetone, ethanol as well as deionized water before use. This AAO template was then placed into the 60 ml Teflon lined stainless steel autoclave and CdTe precursor solution is added to it. The CdTe precursor solutions were prepared by mixing 4 ml of cadmium chloride solution (CdCl₂, 0.04 mol/L) diluted to 42 ml with de-ionized water and sequentially adding, trisodium citrate dihydrate (100 mg), Na₂TeO₃ (0.01 mol/L, 4 ml), 3-mercaptopropanoic acid (MPA) (40.7 mg) and NaBH₄ (50 mg) under magnetic stirring. Then the autoclave was sealed and was kept inside an oven at 180˚C for 4 hours. The oven was programmed to switch off after 4 hours and was allowed to cool to room temperature naturally. The AAO template was taken out of the autoclave, cleaned with de-ionized water and dried with dry N₂ gas.

The luminescence measurements were done with TRIAX 550 monochromatic based Photoluminescence (PL) system with liquid nitrogen cooled InGaAs detector excited by Ar⁺ laser (514.5 nm) with laser power of 10 mW. The topography of the template was taken by SEM. The XRD patterns were recorded on Philips X’pert SW having X’pert pro diffractometer with Cu Kα radiation (α = 1.5406 Å). The sample of AAO template containing CdTe nanorods was made into several pieces and their photoluminescence signature were recorded. These pieces of template with nanorods are then used for sensing different divalent heavy metal ions by physically placing

them in an ionic solution, approximately for 5 minutes and then dried at room temperature. The photoluminescence of these samples were recorded to understand the effect of different divalent ions on the luminescence property of CdTe nanorods. All the optical measurements were performed at room temperature.

Figure 2(a) shows the SEM image of the commercial AAO template after pre-treatment with organic solvents and deionized water used for the growth of CdTe nanorods. The image shows that the template has a precise honeycomb like pore structure with some spongy particulates adsorbed on the surface of the AAO template. The Figure 2(b) shows, the large number of highly distributed CdTe nanorods grown on the AAO template. The di- mension of the rods ranges from 50 - 300 nm in diameter. Figure 3 shows, the XRD of the nanorods grown on the commercial AAO template. The reflections of [111] and [220] planes of Cadmium telluride zinc blend structure are indexed here.

The structure of AAO templates, believes to play an important role in controlling the localization of supersaturated CdTe solution in the pores of the template and have favoured anisotropic growth, which may have led to growth of the nanorods [21] . This can also be the reason that we have highly distributed rods on the AAO template. After a certain critical length, the nanorods might have been dislodged from the base plate to find randomly distributed ones on the template.

The growth procedure of CdTe nanorods can be described as follows. During synthesis, NaBH₄ reduces TeO₃²⁻ to Te²⁻. These Te²⁻ first reacts with Cd²⁺ in the presence of MPA to forms CdTe clusters, which gets deposited in the pores of the AAO template and serves as nuclei for the growth of the nanorods. Increasing Te²⁻ concentration and supersaturation of CdTe in water gives the required kinetic force for anisotropic growth, which leads to the formation of nanorods similar to MnS [21] . The following reaction is proposed in the synthesis of CdTe nanorods on the AAO template [24] .

The template with CdTe nanorods looks brown in color after the reaction and found to exhibit nearly the same emission spectra. Figure 4(a) represents the PL spectra of CdTe nanorods prepared on the AAO template. The PL spectra of CdTe nanorods are broad and asymmetric, which indicated that it consists of more than one component. The CdTe nanorods do not exhibit strong quantum confinement and has an emission band around 759 nm, which is probably a band-to-band emission as the diameter of CdTe nanorods (50 - 300 nm) grown on AAO template is much larger than the diameter of the Bohr exciton in bulk CdTe (10 nm). Figure 4(b) shows the variations in the PL responses of the CdTe nanorods in the presence of 1 µM concentration of different divalent ions such as Pb, Hg, Mg, and Zn. In the presence of Mg²⁺ and Zn²⁺ ions, the PL intensity of the nanorods got enhanced and there was a change in the spectral features too. However, the presence of ions like Hg²⁺ and Pb²⁺ quenches the PL intensity without any change in the spectral features.

The PL quenching and enhancement of CdTe nanorods in the presence of different divalent ions can be due to the modification of surface states or the atoms and molecules located near the surface because of carrier transfer. If the carriers are transferred from the rods to the surrounding ions, the PL intensity decreases or else, if the carrier transfer is in the opposite direction, there is enhancement in PL intensity [25] .

The enhancement and quenching of the photoluminescence can also be possible due to the band alignments of the respective interacting ions species with the rods [26] . The Hg²⁺ and Pb²⁺ ions activate the surface dangling bonds of CdTe and forms narrow band-gap material i.e. HgTe (E_g = −0.3 eV), PbTe (E_g = 0.41 eV ) at the surface. It results in the opening of some non-radiative pathways due to band-alignment (Figure 5) between core-shell

like (CdTe-HgTe, CdTe-PbTe) structures formed due to the presence of the respective ionic species. In this case the shell being low band gap material that results in quenching of PL in CdTe nanorods. The enhancement and red shift in the presence of Zn²⁺ and Mg²⁺ is attributed to the passivation of the surface states due to the similar core-shell structure formed with the higher band gap material as the shell structure CdTe-ZnTe (E_g = 2.26 eV), CdTe-MgTe (E_g = 3.5 eV). The resulting band-alignment is shown in Figure 5. In this case, it is expected that the excitons in CdTe nanorods are efficiently confined inside the large band gap materials (ZnTe and MgTe)

which enhance the PL by preventing non-radiative pathways. Thus, the band alignment between core-shell like structures formed after exposing selective metal ions to the nanorods plays an important role in the luminescence mechanism that helps in understanding the detection of divalent ions here.

In summary, luminescent CdTe nanorods were grown on commercially available AAO (aluminum oxide template) via hydrothermal procedure at a relatively low temperature of 180˚C for 4 h. The structure of AAO templates, believed to participate in controlling the localization of supersaturated CdTe solution in the pores of the template and have favoured the nanorod growth. As obtained CdTe nanorods on the AAO template were used as sensitive and selective probes for the detection of hazardous metal ions like Pb²⁺, Hg²⁺ and other microelements like Zn²⁺, Mg²⁺. The PL of the nanorods quenched in the presence of Hg²⁺ and Pb²⁺ ions however it got enhanced in the presence of Zn²⁺ and Mg²⁺ ions. The enhancement and quenching of PL were described on the basis of band alignments of the respective interacting ion-species. Further investigation is needed to understand the effect of different molar concentration of divalent salts and find out the detection limit.

One of the authors (Gyanan) acknowledges Prof. K. S. R. Koteswara Rao, Department of Physics, IISc for his valuable support in carrying out research work in the laboratory. Also we would like to thank Institute Nanoscience Initiative, IISc, for electron microscopy measurements.