Fabrication and environmental applications of silver coated gold nanorods : electrospun polymer fiber matrix for use as surface enhanced Raman scattering substrates
Date
2018
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Surface-enhanced Raman scattering (SERS), has been recognized as an ultrasensitive
analytical technique since its discovery in the 1970s. After five decades of
development, SERS has been successfully applied in various fields. Despite the great
potential of SERS for trace-level detection, non-destructive testing, and surface
analysis, the lack of reproducibility of SERS substrates has prevented SERS from
being a routine analytical method. To solve this problem, we have proposed a
fabrication strategy for three-dimensional (3D) SERS substrates comprised of SERSactive
metallic nanoparticles (MNPs) and polymeric electrospun fibrous mat. MNPs
are immobilized onto the fibrous mats via electrostatic attraction which acts as a
driving force. Specifically, in this dissertation, silver coated gold nanorods
(Ag/AuNRs) were synthesized and used as SERS-active MNPs. To utilize nonspecific
electrostatic attraction as the driving force to direct the immobilization of
Ag/AuNRs, opposite electrostatic charges were introduced on Ag/AuNRs and
electrospun polycaprolactone (PCL) fibers. Negative charges were introduced on
Ag/AuNRs surface by coating a layer of polyanion poly(sodium 4-styrenesulfonate)
(PSS), while positive charges on the PCL fiber surfaces were developed through
polyelectrolyte layer-by-layer (LbL) deposition leaving polycation
poly(diallyldimethylammonium chloride) (PDADMAC) as the outermost layer. In the
immersion of LbL modified PCL mat in the PSS coated Ag/AuNRs (PSS/Ag/AuNRs)
solution, PSS/Ag/AuNRs were adsorbed onto PCL fiber surface, where a dense and
uniform decoration of Ag/AuNRs was formed. This fabrication of MNPs/polymer
composite is a generalized method that can be applied to different types of MNPs and
polymer materials. Ag/AuNRs with different silver coating thicknesses and poly (3-
hydroxybutyrate-co-hydroxyhexanoate) electrospun fibers were all used in production
of SERS substrates. ☐ Regarding the sensitivity, reproducibility and stability, SERS performance of
the Ag/AuNRs-PCL composites was evaluated using 4-mercaptopyridine (4-Mpy) as a
probe molecule. A limit of detection (LOD) as low as 10-8 M and excellent
reproducibility with less than 7% intensity variations among nine measurements were
achieved. In addition, the SERS substrate had good long-term shelf stability. The
SERS intensities obtained from a 3-month aged substrate had negligible differences
when compared to the intensities obtained from a freshly prepared substrate.
Enhancement factor, an enhancement indicator of SERS substrates, compared to
normal Raman, was calculated to be 104-105 for the major peaks of 4-MPy.
Comparison experiments were conducted using Ag/AuNRs-PCL substrates having
different silver coating thicknesses and AuNRs-PCL fabricated using the same
strategy. The results showed that the Ag/AuNRs-PCL substrates with different silver
coating thickness had overall better SERS sensitivities than AuNRs-PCL substrate,
especially for SERS-sensitive vibration modes of 4-Mpy. This could be attributed to
the larger surface area of Ag/AuNRs compared to AuNRs allowing a higher loading of
4-Mpy molecules and the charge transfer between silver and gold in the bimetallic
Ag/AuNRs to enhance the local electric field of Ag/AuNRs. ☐ To explore the potential application of Ag/AuNRs-PCL SERS substrates in
environmental sensing, we have carried out successful speciation and quantification of
several arsenic species. Ag/AuNRs provide appropriate surface chemistry for arsenic
analysis through the formation of As-O-Ag bonds. A LOD as low as 4 ppb for various
inorganic and organic arsenic species was achieved. Moreover, Ag/AuNRs-PCL
substrates have been applied to investigate arsenic activities and molecular
configuration on a silver surface. In-situ SERS observation of arsenic adsorption,
desorption and reduction have been monitored. A detailed analysis of the SERS
spectra of p-arsanilic acid (pAsA) under different concentrations and pH conditions
provided insights on the nature of the binding structures of pAsA on a silver surface.
pAsA- and pAsA2- are the pAsA tautomers that can effectively be adsorbed onto the
silver surface but have different molecular structure interacting with the surface.
pAsA- was speculated to bind onto the silver surface in a tilted fashion while pAsA2-
was the tautomer that binds with a 90-degree angle to the silver surface and
contributes most of the observed SERS intensity due to its perpendicular orientation
and chemical interaction with the Ag surface. These spectra studies were supported by
theoretical calculations for the bonding configurations of pAsA- and pAsA2- on the
Ag(111) surface executed by Dr. Fernando Sabino and Dr. Anderson Janotti. All the
results are strong evidence that the 3D Ag/AuNRs-PCL substrate is of great potential
for SERS measurements and serving as an environmental sensor for arsenic analysis.