A new and facile route to prepare gold nanoparticle clusters on anodic aluminium oxide as a SERS substrate

Highlights

• A new and facile route was developed to prepare SERS substrate without any need for complex production step.

• Flower-shaped gold nanoparticle clusters were produced on the anodic aluminium oxide.

• Label free SERS measurements of nitrate ion were performed on the developed sensing platform.

• High sensitivity and good recovery for the nitrate ion in drinking water samples was obtained.

Abstract

In the present study, a new SERS-active gold nanoparticle clusters having a flower-shape have been prepared easily on nano porous anodic aluminium oxide (AAO) by immersing it in auric chloride solution without any need for complex production steps. In this process, presented for the first time, the metallic aluminum which were released under the influence of chloride ions due to pitting corrosion act as a reducing agent, while gold ions were reduced onto the AAO layer based on the difference in standard reduction potentials between aluminum and gold. Gold nanoparticle clusters on AAO layer formed “hot spots” providing enhanced Raman signal. Optical microscope, SEM, EDX, AFM, and UV–vis spectrophotometer have been used to characterize the substrate. In order to demonstrate applicability of the method, label free SERS measurements of nitrate ion was performed on the proposed sensing platform. A high sensitivity with 1.03 ppm of limit of detection level and the enhancement factor of 2.9 × 10⁵ were obtained for nitrate ion. In addition, remarkable recoveries ranging from 98.4% to 106.8% were obtained for nitrate spiked into drinking water samples. The inter-day and intra-day precisions of the method as relative standard deviation (RSD) were determined as 3.3% and 5.2%, respectively. The sensor platform, developed using a facile method and a low-cost base material (aluminum), can be a good alternative for SERS based sensing applications.

Introduction

Due to its capability of highly sensitive and rapid detection, SERS method has been widely used to detect organic and inorganic molecules in many areas such as medicine, environmental monitoring, homeland security, and food safety [1,2]. In SERS detection, two mechanisms, electromagnetic (EM) and chemical, can be defined to improve the Raman signal. EM enhancement can be explained with the amplification of localized electromagnetic field as a result of the excitation of surface-plasmon resonance by EM wave at or very close to metallic nanostructured substrates, while chemical enhancement depends on the charge transfer between the noble metal and the adsorbed analyte [3]. Since the enhancement of Raman signal is almost universally accepted to be based on EM theory, most of the studies on SERS substrate have been focused on the EM mechanism [[4], [5], [6]]. In fact, more recent studies attribute SERS enhancement to hotspots localized due to surface roughness and aggregated nanoparticles rather than nanoparticle surface plasmons [7]. Further signal enhancement can be obtained through to the formation of rough surfaces as gaps, cavities, islands, and crevice in nanometer size [8].

Gold nanoparticles have been commonly used for SERS based applications due to their unique optical properties [9]. A variety of methods are used for preparing SERS substrate including electrodeposition, electroless deposition, and nanosphere lithography, as well as e-beam lithography and nanoimprint lithography [[10], [11], [12], [13]]. Another method involves the use of gold nanoparticles in different shapes in a solution and their assembly on a solid substrate [7,14].

Among the substrates, AAO has been proved to be a very good SERS platform with many superior properties including its organized nanostructure, large surface area, cost-competitiveness, and scalable fabrication process. AAO based SERS platforms are generally prepared by spraying or evaporating gold and silver onto the surface of AAO substrate. Apart from these, there are some applications such as decorated metal nanoparticles, embedded nanowires, and metallic membranes [[15], [16], [17]]. AAO substrates are prepared using anodizing method. In the anodizing process, aluminum is used as the anode, while platinum, gold, carbon or aluminum can be used as the cathode. Sulphuric acid (H₂SO₄), oxalic acid (H₂C₂O₄), and phosphoric acid (H₃PO₄) are generally used as electrolytes. By changing the anodizing parameters (e.g. electrolyte type, voltage, or exposure time), pore diameter and pore depth can be adjusted [18]. Moreover, AAO is one of the topics in corrosion studies. AAO layer has been reported to increase the hardness and wear resistance of Al alloys as well as their corrosion resistance [19,20]. Metal weariness occurring locally on the aluminum surface having a natural oxide layer has been explained by the pitting corrosion phenomenon [21]. With this mechanism, aluminum was activated and various metal nanostructures and alloys were developed by using wet chemical method. The pitting corrosion method of aluminum has been described as a strategy for the synthesis of metal nanoparticles [22].

The constant drinking of water which contains high levels of nitrate causes throat and blood diseases and even death. When infants are exposed to concentrated nitrate in water, nitrate is reduced to nitrite by intestinal bacteria. These nitrites bind to hemoglobin in the red blood cell, forming methemoglobin that cannot carry oxygen. This problem causes a type of poisoning called blue baby syndrome in which baby's skin turns into blue [23,24]. Various analytical methods are applied for the detection, determination, and monitoring of nitrate concentration in water. Although most of the laboratory-based techniques provide highly sensitive nitrate detection, special equipment and sample preparation procedure is required for their operation. The most commonly used spectroscopic methods include UV/vis, chemiluminescence, fluorimetry, IR, Raman, and molecular space emission. Among these techniques, Raman spectroscopy is advantageous in that it enables rapid detection and identification of analyte in a very small volume though it causes low sensitivity due to weak signal phenomena [[25], [26], [27]]. With the development of SERS based on metal nanoparticles, the weak signal problem has been largely overcome [28,29].

SERS method offers many superiority such as the use of samples in very small volumes, highly sensitive, selective and rapid detection of chemicals, as well as facilitating on-site analysis with portable Raman devices due to its features suitable for miniaturization [30,31]. However, each of the wet chemistry techniques used to produce SERS substrate has some difficulties to overcome, such as non-reproducibility, long-time production procedure, instability and strong background signal. Despite lithography techniques provide reproducible production and stability with periodic metal structure, high technology and expensive facilities are required [32]. Therefore, there remains a need to develop low-cost, reproducible and facile methods to produce the SERS substrate. In this study, we offer a new, simple, and low-cost route to prepare a SERS substrate. In the anodizing process, AAO layer was formed on Al to be used as a substrate. Flower-shaped gold nanoclusters were created on AAO by immersing it in auric chloride solution without using any special equipment. In addition, high-sensitivity, reproducible and facile label-free SERS based nitrate detection in drinking water was demonstrated on the developed SERS platform.