Branching and size of CTAB-coated gold nanostars control the colorimetric detection of bacteria

Abstract

COMMUNICATIONRapid detection of pathogenic bacteria is challenging because conventional methods require long incubation times. Nanoparticles have the potential to detect pathogens before they can cause an infection. Gold nanostars have recently been used for colorimetric biosensors but they typically require surface modification with antibodies or aptamers for cellular detection. Here, CTAB-coated gold nanostars have been used to rapidly (<5 min) detect infective doses of a model gram-positive pathogen Staphylococcus aureus by an instrument-free colorimetric method. Varying the amounts of gold nanoseed precursor and surfactant can tune the size and degree of branching of gold nanostars as studied here by transmission electron microscopy. The size and morphology of gold nanostars determine the degree and rate of color change in the presence of S. aureus. The optimal formulation achieved maximum color contrast in the presence of S. aureus and produced a selective response in comparison to polystyrene microparticles and liposomes. These gold nanostars were characterized using UV-Visible spectroscopy to monitor changes in their surface plasmon resonance peaks. The visual color change was also quantified over time by measuring the RGB components of the pixels in the digital images of gold nanostar solutions. CTAB-coated gold nanostars serve as a promising material for simple and rapid detection of pathogens. IntroductionGold nanostars are an interesting class of materials because of their excellent performance in colorimetric biosensors, [1][2][3][4] surface enhanced Raman spectroscopy (SERS), [5][6][7][8][9][10][11] imaging and therapy, 12,13 as well as recently in solar cell power conversion. 14 The optical and electrical characteristics of gold nanostars are governed by their size and degree of branching. 11,15 Hence, control over these parameters is essential and has previously been demonstrated using methods such as seed-free growth, 16 the use of poly(vinylpyrrolidine) (PVP), 11,17,18 and even surfactant-free synthesis, but a systematic study of seedmediated synthesis assisted by the surfactant, cetyltrimethylammonium bromide (CTAB) is lacking. The use of nanoseed precursor and surfactant offers the opportunity to control the size and degree of branching of the nanostars using these two simple parameters. The morphology of nanostars determines the peak of light absorption and hence the color of gold nanostars. The peak of absorption in gold nanoparticles changes with their aggregation state. The shift in this peak causes a drastic color change that is detectable by the naked eye and is ideal for application in a biosensor. Furthermore, nanoparticles have increased kinetics in solution when compared with their microparticle counterparts, suggesting that rapid detection may be feasible using a biosensor platform at the nano-scale. 19,20 Food poisoning continues to cause severe illness around the world and leads to hospitalization of unsuspecting patients. The concentration of pathogens necessary for successfully ...

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