Facile Microwave Assisted Synthesis of Silver Nanostars for Ultrasensitive Detection of Biological Analytes by SERS

Abstract

We report a very simple, rapid and reproducible method for the fabrication of anisotropic silver nanostars (AgNS) that can be successfully used as highly efficient SERS substrates for different bioanalytes, even in the case of a near-infra-red (NIR) excitation laser. The nanostars have been synthesized using the chemical reduction of Ag⁺ ions by trisodium citrate. This is the first research reporting the synthesis of AgNS using only trisodium citrate as a reducing and stabilizing agent. The key elements of this original synthesis procedure are rapid hydrothermal synthesis of silver nanostars followed by a cooling down procedure by immersion in a water bath. The synthesis was performed in a sealed bottom flask homogenously heated and brought to a boil in a microwave oven. After 60 s, the colloidal solution was cooled down to room temperature by immersion in a water bath at 35 °C. The as-synthesized AgNS were washed by centrifugation and used for SERS analysis of test molecules (methylene blue) as well as biological analytes: pharmaceutical compounds with various Raman cross sections (doxorubicin, atenolol & metoprolol), cell lysates and amino acids (methionine & cysteine). UV-Vis absorption spectroscopy, (Scanning) Transmission Electron Microscopy ((S)TEM) and Atomic Force Microscopy (AFM) have been employed for investigating nanostars' physical properties.

Keywords: SERS; anisotropic silver nanostars; bioanalytes; cell lysates; pharmaceutical compounds.

Figure 1

UV-Vis absorption spectra of the colloidal solution containing AgNS. The inset shows an optical image of the colloidal solutions before (a) and after purification through centrifugation (b) respectively.

Figure 2

TEM image of a typical individual silver nanostar (a). TEM image of the central zone of the nanostar (b).

Figure 3

STEM image of an individual silver nanostar (a). STEM image of the central zone of the nanostar (b).

Figure 4

AFM topological image of several interconnected nanostars self-organized on the ${\mathrm{MgF}}_2$ surface (a). Corresponding cross-sectional analysis of AgNS (b). The scale bar represents 2 µm.

Figure 5

High resolution AFM topographical image of silver nanostars. The scale bar represents 2 µm.

Figure 6

Raman spectra of AgNS recorded using a 785 nm laser excitation.

Figure 7

SERS spectrum of dried MB solution (1 µM) using a 785 nm excitation laser (a). Raman spectrum of dried MB solution (1 µM) using a 785 nm excitation laser (b).

Figure 8

SERS spectrum of dried DOX solution (1 mM) collected on an isolated nanostar using a 785 nm excitation laser (a). The inset shows an optical image of the individual nanostar that has been used as plasmonic substrate. Raman spectrum of dried DOX solution (1 mM) collected using a 785 nm excitation laser (b).

Figure 9

SERS spectrum of MET 1mM (a) collected on individual AgNS using a 785 nm laser. The inset in figure (a) shows an optical image of the individual nanostar that has been used for recording the spectra. Raman spectrum of MET 1 mM (b) collected using a 785 nm laser.

Figure 10

SERS spectrum of ATE 1 mM (a) collected on individual AgNS using a 785 nm laser. Raman spectrum of ATE 1 mM (b) collected using a 785 nm laser.

Figure 11

SERS spectra of DLD1 cell lysate obtained using a 785 nm excitation laser (a). Raman spectra of DLD1 cell lysate obtained using a 785 nm excitation laser (b).

Figure 12

SERS (a) and Raman (b) spectra of Cysteine (1 mM) recorded using a 785 nm excitation laser.

Figure 13

SERS (a) and Raman (b) spectra of Methionine (1 mM) (a) recorded using a 785 nm excitation laser.