A green synthesis of gold–palladium core–shell nanoparticles using orange peel extract through two-step reduction method and its formaldehyde colorimetric sensing performance

Highlights

• Core–shell nanoparticles could be simply prepared using orange peel extract.

• Orange peel extract acts as both reducing as well as stabilizing agent for synthesis of Au@PdNPs.

• Au@PdNPs have a spherical shape with Au core size of 40 nm and a 7 nm of Pd shell.

• Au@PdNPs exhibit a high sensitivity as a signal transducer for the colorimetric detection of formaldehyde.

• The colorimetric sensing system easily observed both by naked eye or using spectrophotometer with a good linearity, repeatability, and selectivity.

Abstract

Gold–palladium core–shell nanoparticles (Au@PdNPs) have been successfully synthesized through a two-step facile and green method. HAuCl₄ and H₂PdCl₄ were used as the precursors, while orange peel extract (OPE) was used as both reducing and stabilizing agent. Initially, OPE was used to synthesize the gold core nanoparticles (AuNPs), followed by the reduction of PdCl${}_4^{2-}$ solution to produce the Pd shell. The synthesized nanoparticles showed a strong visible absorbance at 534 nm indicated the formation of AuNPs at an optimum water/OPE ratio of 16.67, while the formation of Au@PdNPs was indicated by the disappearance of the peak at 534 nm. Further characterization by using IR spectroscopy indicated that –OH groups of OPE plays the main role in the nanoparticles formation, while TEM images observed a core–shell structure of the Au@PdNPs with an average core diameter of 40 nm and an average shell thickness of 7 nm. In the presence of different formaldehyde concentrations, the core–shell nanoparticles showed a clear color change from light to dark brown. Linearity of the absorbance responses from the formaldehyde concentration range of 36.3 mM to 3.63 M (R² = 0.991) was achieved with an estimated LOD of 304.9 mM, suggested that the developed nanoparticles are promising to be applied for formaldehyde sensing.

Introduction

Bimetallic nanoparticles have attracted many researchers due to the specialty of catalytic, optical, magnetic, and electrical properties compared to their monometallic nanoparticles [1], [2], [3]. One of the most fascinating bimetallic nanoparticles is gold–palladium with the core–shell structure (Au@PdNPs). The Au@PdNPs has been successfully employed as electrocatalyst for antibiotics sensing [4], selective organics redox reaction [5], [6], fine chemicals production [7], [8], and colorimetric sensing [9], [10].

Various synthesis methods of Au@PdNPs have been reported, such us galvanic replacement reaction [6], seed-mediated growth [3], thermal reducing [5], sol-generation [11], thermal decomposition [8], anion coordination [12], reserve micelle [13], co-chemical reduction [14], sonochemical irradiation [15], and bioreduction (green synthesis) methods [2]. Among these methods, green synthesis using plants and plant-derivative extracts have been attracted many interests due to the eco-friendly, good stability, less toxicity, rapidity, and readily conducted at ambient condition [16], [17]. In addition, plant extract contains chemical components which could act both of reducing and stabilizing agents [18].

One of many potential plant-derivatives to synthesize the Au@PdNPs is orange (Citrus sinensis) peel waste which contains high reductive compounds, such as aldehydes and citrates [19] as well as phenolic and flavonoid groups [20], [21]. It also contains a high amount of ascorbic acid [22] and citric acid which are chemical substances that commonly used as stabilizing (capping) as well as reducing agents in the nanoparticle synthesis [23]. Moreover, both agriculture and domestic industry produce a huge amount of waste, including the orange peel. Therefore, the utilization of this waste is desired. The utilization of orange peel extract (OPE) to synthesize silver [19], [20], platinum [24], and zinc oxide nanoparticles [25] have been reported. However, the utilization in the synthesis of bimetallic core–shell nanoparticles is yet to be explored.

Metal-based such as gold, silver, platinum, and copper-nanoparticles have been widely applied in the colorimetric sensing due to the unique optical, high extinction coefficient, and physico-chemical properties, which allow them as sensitive and selective probes for various chemical and biological samples [26], [27], [28], [29]. On the other hand, several researchers have reported the utilization of Au@PdNPs in colorimetric detections, such as for Campylobacter jejuni cells [9] and glucose [10]. Unfortunately, utilization of the Au@PdNPs for formaldehyde colorimetric detection has not been reported. Globally, formaldehyde (about 37% w/v $\sim$13.43 M) was illegally used as a preservative agent for processed foods that caused pain, vomiting, seizures, and death, if consumed in large quantities [30], [31]. International Agency for Research on Cancer (IARC) classified formaldehyde to group 1 carcinogenic agent for humans [32]. Various methods have been employed for formaldehyde detection, including spectrophotometry [33], gas chromatography–mass spectrometry (GC–MS) [34], high-performance liquid chromatography (HPLC) [35], voltammetry [36], [37], and colorimetric methods [38]. Among those methods, the colorimetric method offers advantages such as simplicity, low-cost, rapidity, as well as no required special instrumentation and qualification, compared to the other methods [29], [38]. Therefore, taking advantages of the green synthesis method [39], it is very fascinating to develop formaldehyde colorimetric sensing using the Au@PdNPs that prepared via the green synthesis route.

Recently, Ding et al. [3] reported the synthesis of Au@PdNPs via a seed-mediated method with a huge consumption of chemicals, including cetyltrimethylammonium bromide, NaHB₄, and ascorbic acid, which are potentially harmful to the environment. In this work, the green synthesis of bimetallic nanoparticles Au@Pd was performed, involving two simple reaction steps, i.e. preparation of gold core nanoparticles (AuNPs) and deposition of palladium on AuNPs to be palladium–gold​ core–shell nanoparticles (Au@PdNPs). The effect of OPE concentrations was investigated and detailed characterization including UV–Vis and FTIR spectroscopy, HPLC, XRD, and TEM equipped with EDS were conducted to confirm the properties of the synthesized AuNPs and Au@PdNPs. Furthermore, the performance of the green synthesized Au@PdNPs as a signal transducer (probe) in the colorimetric detection of formaldehyde (HCHO) solution was evaluated.