Engineering gold nanoworms with tunable longitudinal plasmon peak in the near infrared and their refractive index sensing properties.

The plasmonic properties of rod-shaped Au nanoparticles make them promising for numerous applications. The synthesis recipes for Au nanorods are well established and their longitudinal plasmon peak can be tuned over a wide wavelength range. Herein, we demonstrate that the longitudinal plasmon peak of gold NWs (NWs), which are bent nanorods, can be finely tuned in the near-infra-red region. The NWs were synthesized using a one-step reaction method. We have seen that the length and aspect ratio of NWs can be tuned by simply changing the pH of the reaction medium. Under higher pH reaction conditions, NWs with relatively smaller sizes were obtained. Similar to nanorods, NWs have a well-defined longitudinal plasmon peak, which scales linearly with their aspect ratio. Finite element analysis was used to model the optical properties of Au NWs. The simulated results matched well with the experimental spectra. The synthesized NWs have shown good refractive index sensitivities (RIS). The RIS of NWs increased with an increase in their aspect ratio. A maximum sensitivity value of 542 nm per RIU, was obtained for NWs with the plasmon peak at 1033 nm. The RIS values are comparable to that of Au nanorods and bipyramids.

Gold nanoworms: Optical properties and simultaneous SERS and fluorescence enhancement.

Fabrication of platforms for efficient Raman and fluorescence enhancement is very attractive for plasmon-based molecular sensing. For superior sensitivity, the plasmonic constituents of such platforms should be effective electromagnetic field enhancers. Furthermore, nanoparticles having plasmon peak in the spectral range of therapeutic window are superior for biomedical application. Herein, we show that worm-shaped Au nanoparticles can be used for simultaneous surface enhanced Raman scattering (SERS) and metal enhanced fluorescence (MEF). Gold nanoworms (Au-NWs) with an intense plasmon absorption in the therapeutic window were synthesized using a facile single-stepped recipe. Electron microscopy imaging revealed that Au-NWs have a non-uniform surface. Owing to their special morphology, three distinct plasmon bands were seen in the experimental spectrum. The presence of three plasmon peaks was also verified by finite element based simulations. The simulation results further show that Au-NWs can provide intense near field enhancement for multiple excitation wavelengths. As a proof of concept, we have used Au-NWs based platforms for simultaneous enhancement of fluorescence and Raman signal of rhodamine 6G (R6G) dye molecule.

Efficient seed-mediated method for the large-scale synthesis of Au nanorods.

Seed-mediated methods are widely followed for the synthesis of Au nanorods (NRs). However, mostly dilute concentrations of the Au precursor (HAuCl4) are used in the growth solution, which leads to a low final concentration of NRs. Attempts of increasing the concentration of NRs by simply increasing the concentration of HAuCl4, other reagents in the growth solution and seeds lead to a faster growth kinetics which is not favourable for NR growth. Herein, we demonstrate that the increase in growth kinetics for high concentrations of reagents in growth solution can be neutralised by decreasing the pH of the solution. The synthesis of the NRs can be scaled up by using higher concentrations of reagents and adding an optimum concentration of HCl in the growth solution. The concentration of HAuCl4 in the growth solution can be increased up to 5 mM, and 10-20 times more NRs can be synthesised for the same reaction volume compared to that of the conventional seed-mediated method. We have also noticed that a cetyltrimethylammonium bromide (CTAB)-to-HAuCl4 molar ratio of 50 is sufficient for obtaining high yield of NRs.

Facile synthesis of gold nanoworms with a tunable length and aspect ratio through oriented attachment of nanoparticles.

We report a seedless protocol based on the oriented attachment of nanoparticles for the synthesis of Au nanoworms (NWs). NWs are grown by reducing HAuCl4 with ascorbic acid (AA) in high pH reaction medium and in the presence of growth directional agents, cetyltrimethylammonium bromide (CTAB) and AgNO3. Although we have used the same reducing and growth directional agents as typically used for the synthesis of Au nanorods, the growth mechanism of NWs is markedly different from that of nanorods. Instead of the anisotropic growth of seed particles, the NWs grow through oriented attachment of nanoparticles. By varying different reaction parameters we have seen that the length of NWs can be controlled from tens of nanometers to a micrometer. Furthermore, the aspect ratio (AR) can be tuned from 2 to 30. This is almost the whole range of AR and length for Au nanorods so far achieved with seed-mediated multiple step synthesis protocols.

Tuning the oriented deposition of gold nanorods on patterned substrates.

The controlled patterning of anisotropic gold nanoparticles is of crucial importance for many applications related to their optical properties. In this paper, we report that gold nanorods prepared by a seed-mediated synthesis protocol (without any further functionalization) can be selectively deposited on hydrophilic parts of hydrophobic-hydrophilic contrast patterned substrates. We have seen that, when nanorods with lengths much smaller than the width of the hydrophilic stripe are used, they disperse on these stripes with random orientation and tunable uniform particle separation. However, for nanorods having lengths comparable to the width of the hydrophilic stripes, confinement-induced alignment occurs. We observe that different interactions governing the assembly forces can be modulated by controlling the concentration of assembling nanorods and the width of the hydrophilic stripes, leading to markedly different degrees of alignment. Our strategy can be replicated for other anisotropic nanoparticles to produce well-controlled patterning of these nanoentities on surfaces.

Dipole directed ring assembly of Ni-coated Au-nanorods.

We demonstrate the synthesis of novel core-shell nanoring superstructures consisting of non-dipolar Au nanorods coated with magnetic Ni shells. We show that the magnetic dipole induced self-assembly can be tuned by selective reduction of Ni on gold nanorods. Superstructures range from nanorings containing physically separated particles to solid rigid nanorings.

Controlling the morphology of multi-branched gold nanoparticles.

We demonstrate a simple and versatile way to achieve high yield synthesis of shape- and size-controlled multi-branched gold nanoparticles (MBNPs). Control over the shape of the MBNPs was achieved by varying the ratio of gold to the mild reducing agent ascorbic acid, using a seed-mediated growth approach. Higher ascorbate concentrations resulted in the smoothing of branches, leading to the yield of relatively more isotropic particles. Furthermore, we found that using much higher silver concentrations in the growth solution resulted in the formation of rod-shaped micro-features together with MBNPs; we postulate them to be cetyltrimethyl ammonium silver bromide crystals. The as-prepared MBNPs show interesting tunable optical properties that are strongly influenced by the particle shape. The results are discussed in terms of plasmon coupling between the core and branches of the MBNPs.

Quantitative analysis of gold nanorod alignment after electric field-assisted deposition.

We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownian motion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substrate surface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerably improved alignment.