236 Nanoplasmonics
extremely sensitive thermal measurements can be achieved (Sharma et al. 2009).
Similar surface modication approaches can be used to create sensitive, humidity-
based SPR sensors (Weiss etal. 1996). Other interesting examples of using SPR for
measuring physical quantities include real-time thin-lm thickness (Bao etal. 2010;
Santillán etal. 2010) and ow condition monitoring (Iwasaki etal. 2006; Loureiro
etal. 2007). Due to their high sensitivity and instantaneous response to RI change,
SPR sensors normally exhibit better performance compared to most conventional
measurement methods.
8.4.2 chemical sensing
SPR sensors can be used to determine the concentration of both large and small mol-
ecules. As mentioned in the previous section, for pure analytes, SPR can be directly
applied to measure the concentration within a certain range without the need for any
surface functionalization. An excellent example of this is the measurement of aque-
ous glucose concentration in simple nonbiological aqueous media (Zhen and Yi 2001;
Lam etal. 2005). However, in more realistic conditions, often samples are complex
and contain numerous analytes. In this case, surface modication approaches, such as
those described in Section 8.3, will need to be utilized or other techniques that either
result in the direct adsorption of the chemical species onto the SPR-sensing surface or
methods that elicit a secondary chemical reaction affecting the SPR signal.
Many of the applications for chemical sensing are focused on environmental
monitoring and involve detections of compounds such as hydrocarbons, heavy met-
als, dioxins, and various other contaminants found in water supplies (e.g., pesticides,
pharmaceutical agents, etc.) (Homola 2008). In 2007, Farre et al. (2007) demon-
strated an SPR sensor capable of measuring atrazine in water samples at the level of
parts per trillion. This sensor was based on an alkanethiol SAM formed on a gold
surface which was capable of being regenerated. Mauriz etal. (2007) using simi-
lar immobilization techniques demonstrated a two-channel SPR sensor capable of
simultaneous pesticide monitoring of chlorpyrifos, carbaryl, and DDT samples with
sensitivities in the 18–50 ng/L range. Some examples of the use of SPR for gas/vapor
detection are based on adsorption from various hydrocarbons (Abdelghani et al.
1997; Podgorsek etal. 1997) and tetrachloroethene (Niggemann etal. 1996). Miwa
and Arakawa (1996) demonstrated that polyethylene glycohol thin-lm-modied
SPR sensors can provide selective gas detection. Nitrogen dioxide detection was also
illustrated by Ashwell and Roberts (1996) through the use of chemisorption of the
gas molecules using an active SPR gold layer. Chadwick etal. (1994) described the
detection of hydrogen with a surface plasmon approach using a palladium alloy. Most
recently, Herminjard etal. (2009) demonstrated an SPR sensor showing enhanced
sensitivity for CO
2
detection in the mid-infrared range.
In regards to assessing chemical concentration with SPR, three main approaches
are commonly employed. One method is to directly measure the change in the SPR
response following a xed sample injection time over the sensing surface. This type
of analysis is used when the analyte is large enough such that it is capable of pro-
ducing a moderate response even at low molar concentrations. It is also possible
to measure the rate of analyte binding at the beginning of sample injection. If the

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