7 research outputs found
Single Molecule Surface-Enhanced Raman Spectroscopy: A Critical Analysis of the Bianalyte versus Isotopologue Proof
Verification of single-molecule (SM)
detection for surface-enhanced
Raman spectroscopy (SERS) requires the use of two analytes via either
the bianalyte or isotopologue approach. For both approaches, the preferential
observation of the individual analytes over a combination of both
analytes is used to conclude that SM detection has been achieved.
Isotopologues are preferred because they have identical surface binding
affinities and Raman cross sections, whereas bianalyte pairs typically
do not. We conducted multianalyte SERS studies to investigate the
limitations of the bianalyte approach. The bianalyte partners, Rhodamine
6G (R6G-<i>d</i><sub>0</sub>) and crystal violet (CV-<i>d</i><sub>0</sub>), were directly compared, while SM detection
was verified (or disproved) using their corresponding isotopologues
(R6G-<i>d</i><sub>4</sub>, CV-<i>d</i><sub>12</sub>). We found that the significant difference in counts between R6G
and CV can provide misleading evidence for SMSERS. We then rationalized
these results using a joint Poisson-binomial model with unequal detection
probabilities and adjusted the relative concentrations of R6G and
CV to achieve a comparable distribution of SMSERS counts. Using this
information, we outlined the necessary considerations, such as accounting
for the differences in molecular properties, for reliable SMSERS proofs.
Moreover, we showed that multianalyte experiments at the SM level
are achievable, opening the opportunity for new types of SM studies
Single Molecule Surface-Enhanced Raman Spectroscopy without Nanogaps
We provide conclusive proof of single
molecule (SM) detection by
surface-enhanced Raman spectroscopy (SERS) for discrete Ag triangular
nanopyramids prepared via nanosphere lithography (NSL). While the
observation of SMSERS has been well-demonstrated using various chemically
synthesized nanoparticle substrates, they have a high degree of polydispersity
in shape, size, and aggregation state resulting in an interest to
develop more reproducible and uniform nanoparticles. Here triangular-based
nanopyramids were characterized by scanning electron microscopy to
confirm their geometry and interparticle spacing. Then the isotopologue
approach with Rhodamine 6G was used to conclusively prove SM sensitivity
for the individual nanoparticles. NSL’s straightforward, simple
fabrication procedure produces large active arrays. Furthermore, the
tunable localized surface plasmon resonance makes NSL prepared substrates
effective for the detection of resonant molecules by SMSERS
Hydrophobic Collapse Initiates the Poly(<i>N</i>‑isopropylacrylamide) Volume Phase Transition Reaction Coordinate
The best-known examples of smart,
responsive hydrogels derive from
poly(<i>N</i>-isopropylacrylamide) (PNIPAM) cross-linked
polymer networks. These hydrogels undergo volume phase transitions
(VPTs) triggered by temperature, chemical, and/or environmental changes.
PNIPAM hydrogels can undergo more than 50-fold volume changes within
∼1 μs intervals. Studies have tried to elucidate the
molecular mechanism of these extraordinarily large responses. Nevertheless,
the molecular reaction coordinates that drive the VPT remain unclear.
Using visible nonresonance Raman temperature-jump spectroscopy, we
determined the molecular ordering of this VPT. The PNIPAM hydrophobic
isopropyl and methylene groups dehydrate with time constants of 109
± 64 and 104 ± 44 ns, initiating the volume collapse of
PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly
slower, as our group previously discovered (360 ± 85 ns). This
determination of the ordering of the molecular reaction coordinate
of the PNIPAM VPT enables the development of the next generation of
super-responsive materials
Aluminum Film-Over-Nanosphere Substrates for Deep-UV Surface-Enhanced Resonance Raman Spectroscopy
We
report here the first fabrication of aluminum film-over nanosphere
(AlFON) substrates for UV surface-enhanced resonance Raman scattering
(UVSERRS) at the deepest UV wavelength used to date (λ<sub>ex</sub> = 229 nm). We characterize the AlFONs fabricated with two different
support microsphere sizes using localized surface plasmon resonance
spectroscopy, electron microscopy, SERRS of adenine, tris(bipyridine)ruthenium(II),
and trans-1,2-bis(4-pyridyl)-ethylene, SERS of 6-mercapto-1-hexanol
(as a nonresonant molecule), and dielectric function analysis. We
find that AlFONs fabricated with the 210 nm microspheres generate
an enhancement factor of approximately 10<sup>4–5</sup>, which
combined with resonance enhancement of the adsorbates provides enhancement
factors greater than 10<sup>6</sup>. These experimental results are
supported by theoretical analysis of the dielectric function. Hence
our results demonstrate the advantages of using AlFON substrates for
deep UVSERRS enhancement and contribute to broadening the SERS application
range with tunable and affordable substrates