We demonstrated potential features of gold nanoparticle bipyramid (AuNB) for an electrochemical biosensor . The f acile synthesis method and controllable shape and size of the AuNB are achieved through the optimization of cetyltrimethylammonium chloride ( CTAC ) surfactant over citric acid ratio determining the control of typically spherical Au seed size and its transition in to a penta-twinned crystal structure . We observe that the optimized ratio of CTAC and citric acid (CA) facilitates flocculation control in which Au seeds with size as tiny as ~14.8 nm could be attained and finally transformed into AuNB structures with an average length of ~55 nm with high reproducibility . To improve the electrochemical sensing performance of a screen -printed carbon electrode (SPCE), surface modification with AuNB via distinctiv e linking procedures effectively enhanced the electroactive surface area by 40% . Carried out for the detection of dopamine (DA) , a neurotransmitter frequently linked to the risk of Parkinson’s , Alzheimer’s , and Huntington's disease s, the AuNB decorated -carbon electrode shows outstanding electrocatalytic activity that improve s sensing performance , including high sensitivity, low detection limit, wide dynamic range, high s electivity against different analytes, such as ascorbic acid (AA) , uric acid (UA) and urea , and excellent reproducibility. Keywords: bipyramid, AuNP, CTAC, citric acid, surfactant to acid ratio, dopamine, electrochemi cal sensor 1. Introduction Gold nanoparticle (AuNP) is one of the most widely used metal materials as sensing membranes due to its high biological compatibility, large surface -to-volume ratio, high stability, non-toxicity, strong electromagnetic field, and fast electron transfer [1–3]. Either bottom -up or top- down AuNPs -based sensor has raised great attention on enhancing the sensitivity of biochemical sensing for various targets and sensing mechanisms [4–9]. Specifically , in el ectrochemical biosensors signal amplification, the electroactive surface area plays a pivotal role in determining effective sensitivity and specificity due to the increasing binding sites toward the analysis of particular analytes [10]. The carbon -based electrode s modified with AuNP w ere proposed for several applications, such as, DNA hybridization [8], Microcystin -LR[11], glucose [12], hydrogen peroxide [4,13,14] , and other potential applications [15]. The commercially available screen - printed electrode has been a remarkable alternative to conventional glassy carbon electrode (GCE) that requires accompanying instruments , a large solution volume , and a time-consuming and tedious cleaning process [16].The size, shape , and aspect ratio of nanoparticles to modify the electrode are key parameters to improv ing the sensing capability due to their electrocatalytic activity and optical properties[18]. Ren et al. discover ed that sensitivity relied on the surface crystallographic orientations and shape, with greater sensitivity validated with the largest number of Au (111) facets on the surface for the nanoparticles [17]. In another report, Au nano spikes and dendrites were used, formed by electrodeposition with adjustment of directing agent and plating duration. Alternatively, Zeng et al. reported that the catalytic activity of Au nanocag es is greater than that of nano boxes and nanoparticles toward the reduction of p -nitrophenol by NaBH 4 [18]. These studies prove that nanoparticle's dimensional, morphological and topol ogical characteristic s affect the electrochemical response, particularly sensitivity. Gold nanoparticle bipyramid (AuNB) offers distinguished dimension and shape monodispersity among myriads of engineered nanostructures . Moreover, it possesses broadly explorable properties for electroactive surface area enhancement owing to its three -dimensional double -tipsy structure for easier interaction of bioanalytes at the sensing interface. The application of gold bipyramids for optical biosensors ha s been well studied . However, the profound investigation o f its potencies for electrochemical enhancement is not yet widely exploi ted and is an exciting topic in the advancement of the point of care (PoC) unit. Cheng et al. reported a study in modified GCE with gold n anobipyramid/multi -walled carbon nanotube hybrids (AuNB/MWCNTs) to detect dopamine in the presence of ascorbic acid and uric acid with a detection limit of 15 nM [19]. From the side of AuNB synthesis, the seed -mediated growth method has been widely used because it can produce a uniform nano bipyramid. However, controlling the crystal characteristics of the seeds, which ultimately dictates the crystallinity and form of the resultant nanoparticles, remains crucial [20,21] . Many researche rs suggested cetyltrimethylammonium bromide (CTAB) as a stabilization agent for monodispersed gold nanoparticle synthesis [22–24]. The main reason for the CTAB p