One-step fabrication of black gold nanoparticles is presented using sodium citrate as a reducing and capping agent, resembling the standard Turkevich synthesis. Unlike the Turkevich approach, the gold nanoparticles here were synthesized at room temperature, which slows down the reaction rate and therefore the nanoparticle nucleation and growth rate. Moreover, the gold nanoparticles synthesized at the boiling temperature are often characterized by the appearance of a wine-red color within 30 minutes and a narrow plasmon peak centered around 520 nm (Figure ). Herein, at room temperature, a black colloidal solution is gradually observed within 60 minutes with a broadband absorption all over the visible range (400 -800 nm) as shown in Figure as well as a part of the near-infrared region (not shown). The black gold can also transform reflective surfaces (e.g., 5-and 10-cent coins and copper mesh) and flexible surfaces (e.g., filtration paper and cotton substrate) into black absorbing surfaces by a simple drop-casting method, as seen in the insets of the Figure . Broadening the absorption band of the black gold is very important for several applications, where optical excitation with multiple photons is essential. For instance, in ultrafast spectroscopy, such as the surface-enhanced coherent anti-Stokes Raman scattering, multiple laser pulses with different wavelengths should coincide with the absorption band of the gold nanoparticles. Additionally, the resonance between the plasmon band and the excitation wavelength is shown to affect the reaction yield and rate in plasmonic catalysis, which can be easily tuned by broadening the plasmon band. Thus, we here present a plasmonic black colloidal gold with a broadband absorption that would work efficiently for many purposes. Briefly, different volumes of the sodium citrate (Na3Cit) were thoroughly mixed with the gold precursor (Au +3 ) at room temperature in presence of sodium hydroxide. The solution was left undisturbed to allow the assembly and formation of the nanoparticles. The color of the solution changes from yellow to pale gray within 30 minutes and rapidly into dark black. The morphology of the as-prepared particles was first examined by TEM.As shown, the concentration of Na3Cit has a significant effect on the morphology of the obtained particles. Figure shows the formation of an extensive network of gold nanowires obtained using an Au +3 /Na3Cit ratio of 1/0.7 (v/v). The diameter of the individual nanowire is approximately 5 nm. The HRTEM image in Figure shows the lattice spacing of 0.23 nm, which corresponds to the <111> lattice plane of gold. Additionally, the use of different volumes of the Na3Cit did not influence the black color of the colloidal solution. However, the morphology of the particles is affected. When we decreased the amount of the Na3Cit (1/0.25 ratio), the gold nanowires can still be observed but with a high degree of clustering and irregular shapes, as shown in figure a. The lower concentration of Na3Cit and its lower passivation effect might be the reason for this clustering. On the other hand, when a higher amount of Na3Cit is used (1/1), spherical gold particles and short gold chains have been observed (Figure ) since the assembly and the fusion of the initial gold nuclei are somehow deferred. In the absence of NaOH, only brown gold was observed, which is quickly agglomerated in the form of a sponge gold precipitate. Observation of spherical particles when the concentration of the capping agent is changed might present a hint on the mechanism of the nanowire's synthesis. This means that small spherical particles are first formed, which subsequently assemble and fuse to form the nanowires.