Nanosensors in Optics
Nanosensors are devices with
dimensions on the nanometer scale that are capable of monitoring the presence
of a specific chemical or class of chemicals. Nanosensors which employ optical transduction methods are called optical Nanosensors.
Optical nanosensors can
generally be classified into one of two different classes: 1) chemical
nanosensors, or 2) nanobiosensors, depending on the type of recognition element
(chemical or biochemical) used to provide specificity to the sensor. Small
sizes of these sensors allow them to be inserted and precisely positioned
within individual cells to obtain spatially localized measurements of chemical
species in real time.
Fiber optic nanosensors employ
fiber optics that have been tapered on one end to diameters typically ranging
between 20 and 100 nm. Excitons or evanescent fields continue to travel through
the remainder of the tapered fiber’s tip, providing the necessary excitation
energy. Excitation using such a sensor is highly localized, allowing only
species close to the fiber’s tip to be excited.
The most significant
applications of fiber optic nanoprobes to NSOM analyses of biological samples occurred
when a single dye-labeled DNA molecule was detected using near-field
surface-enhanced resonance Raman spectroscopy (NFSERRS). In that work, dye-labeled
DNA strands were spotted onto a surface-enhanced Raman spectroscopy (SERS) substrate
that was prepared by evaporating silver on a nanoparticle-coated surface. Following
preparation of the sample, a fiber optic nanoprobe was raster-scanned over the
sample’s surface, illuminating it point by point, while the resulting Raman
signals were measured with a charge-coupled device (CCD). Based on the
intensity of the Raman signals measured at every location, a two-dimensional
image of the DNA molecules was reconstructed and normalized
for surface topography based on the intensity of the Rayleigh scatter.
for surface topography based on the intensity of the Rayleigh scatter.
FIBER OPTIC CHEMICAL NANOSENSORS
Fiber
optic chemical nanosensors have chemical recognition elements (e.g.,
fluorescent indicator dyes, etc.) bound to the tapered tip of the fiber to
provide a degree of specificity. It is important to employ a sensitive detection
system, such as the one shown in the following figure.
 |
| Fiber Optic Scanner |
In
such a system, the sample is excited by launching an intense light source
(e.g., laser) into the proximal end of the fiber optic nanosensor. The
nanosensor is then positioned in the desired location using an x–y–z micromanipulator
or piezoelectric positioning system mounted on a microscope. Once in place, the
fluorescent indicator dye immobilized on the tip of the fiber is excited, and
the resulting fluorescence emission is collected and filtered by the microscope
before being detected with either a photomultiplier tube (PMT) or a CCD.
One
advance in the last several years has been the development of
nanoparticle-based optochemical sensors, with nanometer-scale sizes in all
three dimensions. Because of the small sizes of these sensors, a large number
of them can be implanted within an individual cell at one time, allowing for
the monitoring of many locations simultaneously. Although many different
nanoparticle-based sensors are currently being developed, three main classes
have already shown a great deal of promise for intracellular analyses. These
three classes are
·
Quantum
dot-based nanobiosensors
·
Polymer-encapsulated
nanosensors known as PEBBLEs
·
Phospholipid-based
nanosensors