## 1 Introduction  
Recently, nanomaterials have drawn much attention due to their advanced physical and chemical properties over bulk materials, and variety of applications in the biomedical, environmental, industrial, and food agriculture industries [1]. Among nanomaterials, gold nanoparticles with nontoxicity and high bioactivity are the most stable metal nanoparticles, and they present a wide range of applications in modern medical and biological studies such as drug, deoxyribonucleic acid (DNA), and antigens delivery; biosensors, genomics, immunoassay, and detection of microorganisms and cancer cells; and bioimaging [2; 3]. The nanoparticles present their unique physical and chemical properties, particularly optical surface plasmon resonance (SPR), which is related to collective excitations of conductive localized electrons on surface metals under visible and near infra-red (VIS-NIR) irradiation [4; 5; 6; 7]. In fact, SPR effects have been used to enhance the surface sensitivity of several spectroscopic analyses including infra-red/ultra-violet-visible (IR/UV-Vis) light absorption or deflection spectroscopy, hyper-Rayleigh, differential static, and dynamic light scattering, as well as surface-enhanced Raman scattering (SERS) [8; 9; 10; 11; 12]. Recently, gold nanoparticles have been studied for application in SERS-based analytical chemistry due to their remarkable SPR [13]. In conventional Raman-supported analysis methods, the intensity of Raman scattering is relatively weak despite high intensity of the applied incident laser beam, resulting in low sensitivity of the method [14]. However, in the presence of gold nanoparticles, when the analyte molecules are adsorbed onto the surface of gold nanoparticles, the enhancement of the electromagnetic field on the surface of the nanoparticles due to SPR will increase the Raman scattering intensity, thus significantly improving the sensitivity of the method [15; 16]. It has been argued that the shape, size, and structure of gold nanoparticles determine their chemical and physical properties [17; 18]. To date, possible wet synthesis methods have been used to fabricate gold nanoparticles of various controlled sizes, shapes, and structures such as nanospheres, nanorods, nanoshells, nanocages, nanostars, and so forth [19]. Among different anisotropic shapes of gold nanoparticles, the use of gold nanorods (AuNRs) as SERS substrates has gained much attraction due to their remarkably aspect-ratio-dependent plasmonic properties [20]. In addition to their transverse plasmon absorption, the longitudinal absorption of AuNRs, which is related to the oscillation of electrons along longitudinal axis, can be tuned easily by changing their aspect ratio (AR, length/width ratio) [21]. Then, the surface Raman scattering enhancement from AuNRs can be optimized by adjusting the shape and size of nanorods. The most popular protocol to synthesize AuNRs with uniform size and aspect ratio is a seeded growth route pioneered by [PERSON] and coworkers [22]. The method was performed with the aid of cetyltrimethylammonium bromide (CTAB) and ascorbic acid as a surfactant and a reducing agent, respectively, during the nanorod growth in the presence of silver ions. However, the obtained AuNRs usually have poor quality, poor reproducibility, and low aspect ratios with LSPR \(<\)850 nm [23]. In order to reproducibly achieve AuNRs with a high aspect ratio and high quality, [PERSON] et al. replaced ascorbic acid with hydroquinone, an acromatic molecule with a weak reducing power [24]. The use of hydroquinone could result in a mild reaction condition of nanorod synthesis which is less dependent on reagent concentration, therefore increasing the reliability of the method [24]. By varying the hydroquinone, gold, silver, and seed concentration in the growth solution, it is possible for them to fabricate AuNRs with LSPR up to 1230 nm, and with an aspect ratio of 6-7. Later on, another research group led by [PERSON] also reported a successfully controlled hydroquinone-based synthesis of AuNRs.