230 Nanoplasmonics
8.3 OPTICAL-CHEMICAL TRANSDUCING MEDIUMS FOR SPR
Although SPR is very sensitive to subtle changes in the RI of the media in the vicinity
of the metal-sensing surface(s), SPR by itself is not a selective technique. Therefore, a
key component to develop useful biological and chemical SPR sensors is integrating
an optical-chemical transducing medium to the sensor. This normally entails immo-
bilization of biomolecular-recognition elements achieved through a chemical modi-
cation (e.g., aptamer, antibody, molecular imprinted polymer, etc.) at the metal–media
interface (Hoshino etal. 2008; Kim etal. 2010; Ouellet etal. 2010; Zheng and Cameron
2011; Zheng etal. 2011). The surface modication serves multiple purposes. The most
obvious of which is that it allows the surface to selectively capture the desired analyte
in the region for which the SPR interaction occurs. Capturing or adsorbing the target
also enhances the relative RI change at the surface, thereby further improves the
sensitivity. This is especially important for applications involving small molecules
present at low concentrations. To date, most commercial SPR biosensors are capable
of detecting in the order of 1 pg/mm
2
of absorbed analytes of which surface func-
tionalization plays an integral role. The sensitivity for SPR systems is normally indi-
cated in terms of detectable RIU changes, and most commercial systems have RIU
sensitivities between 10
5
and 10
6
. In addition to sensitivity considerations, through
optimizing the surface chemistry, it is possible to avoid other nonspecic interactions,
such as undesired protein adsorption and/or to repel other molecules that may lend to
measurement error or false-positive detections.
8.3.1 self-assembled mOnOlayers
Self-assembled monolayers (SAMs) are organized layers of immobilized molecules
that when prepared can be highly stable, densely packed, and ordered. Their struc-
ture begins with one end of a specic molecule known as the head group. This group
is often specic and has a reversible afnity for a certain substrate which is often a
noble metal. The secondary linkage, normally an alkane chain, serves as a spacer.
The purpose of the spacer is to provide a well-dened thickness as well as acting as
a physical barrier. It is also capable of altering the electronic conductivity as well
as other local properties. Connected to the spacer is the terminal or tail group (e.g.,
OH, –NH
3
, or COOH groups). These groups enable specic binding for certain
molecules, such as antibodies, aptamers, or other attachments. These will be dis-
cussed in the subsequent sections. To date, the most common substrate employed,
especially in SPR sensing, is gold as there are many possibilities that allow for differ-
ent structural variations. As for head groups, thiols (SH) are commonly used with
gold due to their selective binding and they also allow for the incorporation of a wide
range of functional groups or spacers.
In 2010, Kumbhat etal. (2010) demonstrated a SAM-based covalent immobiliza-
tion of a bioreceptor conjugate of a dengue antigen and bovine serum albumin for
SPR. Other recent examples of applying SAMs for SPR sensing include a peptide
SAM (Bolduc etal. 2010, 2011), a double-layer SAM for supporting DNA in SPRi
(Chen etal. 2009b), and a double-layer SAM for attaching antibodies (Lehnert etal.
2011). Even though SAMs can enable selective binding effects, it is well recognized

Get Nanoplasmonics now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.