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4…Advances in SensorSensor Design

4…Advances in SensorSensor Design

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R. C. Evans and P. Douglas

real-life sensing environments. Less focus is being placed on the now wellestablished chemical and/or physical principles behind optical sensing and instead,

efforts are directed at advances in device design, performance and field-viability,

driven by the wide availability of low-cost electronic components, smart materials

and improved detection techniques.

Fibre optic technology is widely used. Optical fibres are made from a cylindrical core surrounded by a cladding layer; both components are made from

dielectric materials but with different refractive indices. If light enters the core

within a critical angle, hc, it will undergo total internal reflection as it travels down

the fibre. Energy is conserved at the core/cladding interface; however a portion of

the electromagnetic field component, known as an evanescent field wave, penetrates a short distance (*20 nm) into the cladding. The interaction of this evanescent wave with an analyte may be used as a sensing platform. Removal or

tapering of the fibre cladding exposes more of the evanescent field, such that it can

be absorbed by, scattered by, or excite molecules located at the fibre surface. If the

evanescent wave is generated in close proximity to a thin metallic layer, cooperative interaction with surface plasmons can lead to an enhanced effect known as

surface plasmon resonance (SPR). Surface plasmons are collective oscillations of

free electrons occurring at the interface between a metal and a dielectric. If a thin

layer of a noble metal (e.g. Ag, Au) is inserted at the core/cladding interface, the ppolarised component of the evanescent field may excite surface plasmons in the

metal layer. SPR sensors are extremely useful for studying adsorption events at

surfaces, such as protein adsorption and biorecognition events [43].

12.4.1 Integrated Sensor Devices

Technological advances have made integration of the sensing element, excitation

source, detector and electronics within a single miniaturised device possible. The

most common semi-integrated devices are fibre optic sensors, or optodes, where

the sensor layer is either coated on the tip of an optical fibre (for absorbance and

luminescence sensing) or on an unsheathed section of the fibre (for evanescence

wave sensing) [3]. Many fibre optic sensors are now commercially available (e.g.

for oxygen [44]). The major advantage of fibre optics is that they enable light to be

carried long distances, so that optodes can be used in remote or hazardous sensing

environments, which may be inaccessible by other analytical techniques. Nonetheless, a separate light source and a detector to quantify the signal, are still


However, advances in miniaturised electronics mean that fully-integrated

optical sensor devices are now a real possibility. Hand-held portable fluorescence

spectrometers are now commercially available, enabling optical sensing to be

taken out of the laboratory [44]. Low-cost inorganic light-emitting diodes (LEDs)

emitting across the UV/Vis/NIR spectral region are already common excitation

sources. These overcome the size, geometrical and electronic restrictions imposed


Optical Sensors and Probes


by traditional excitation sources such as lamps and lasers. The development of

organic LED (OLED) technology has led to the development of truly integrated

optical sensor platforms. Thin film p-i-n photodiode detectors can now be easily

fabricated on glass or plastic substrates. Integrated sensor devices incorporating an

OLED excitation source and a photodiode detector, and based on a luminescence

quenching mechanism for the detection of a variety of analytes (e.g. O2, glucose,

ethanol) have been reported [45]. In these devices the sensor layer is deposited on

one side of a transparent substrate, and the OLED excitation source is fabricated

on the other side. The photodetector is placed behind the transparent OLED.

Luminescence from the sensing layer transmits through the transparent layer and is

measured by the photodetector. The availability of multi-pixel OLED arrays,

suitable for use with a variety of lumophores, makes these compact devices ideal

candidates for sensor microarrays for multi-analyte detection.

12.4.2 The Sensor Layer

Immobilisation of the optical probe in a permeable host matrix is a critical part of

the sensor design process. The support matrix of choice varies between sensors,

depending on the application requirements, and the final decision is usually based

on the following considerations:

probe-host compatibility;

optical properties of the host e.g. refractive index, transparency;

permeability of host to analyte;

tendency for leaching (particularly for solution phase sensing).

The most commonly used host materials are polymers [46] and sol–gels [47].

Typical polymer hosts include organic glassy polymers (e.g. polystyrene), fluoropolymers, and cellulose derivatives. For polymers with a high glass transition

temperature, a plasticiser is often added to make them more flexible and to

improve analyte permeability. Sol–gel films are usually based on organically

modified siloxanes (Ormosils), in which the surface Si–OH groups are replaced by

Si-R groups (e.g. R = methyl, ethyl), rendering the surface hydrophobic. Ormosils

are therefore suitable hosts for ambient and dissolved gas sensors.

Preparation of the sensor layer is relatively straight-forward: a known quantity

of the probe is mixed with the polymer or sol–gel solution, which is then cast as a

thin film via spin-coating, dip-coating or screen printing. The simplicity of this

approach means that it is possible to add additional components to the layer, such

as a reference dye for internal calibration or a scattering agent to improve sensitivity. The main disadvantage is that it is very difficult to ensure that the probe is

homogeneously dispersed through the host layer. This can lead to a distribution of

probe sites within the layer, which result in a variation in both the optical properties of the probe and its interaction with the analyte.


R. C. Evans and P. Douglas

‘PEBBLE’ sensors (Probes Encapsulated By Biologically Localised Embedding), consisting of one or more probes trapped in cross-linked polymer beads,

have been developed as sensors for intracellular use in conjunction with fluorescence microscopy [48]. Intracellular cation and anion sensing is of great biological

importance, but often, biomolecules will preferentially bind to the probe over the

target analyte. The high-degree of cross-linking in the beads therefore serves to

both trap the probe and to prevent biomolecules from entering the beads, whilst

still allowing the permeation of small anions and cations.

12.4.3 Calibration and Amplification of the Sensor Response

In real analytical situations the sensor response for a specific analyte often suffers

from interference from other environmental parameters. Moreover, fluctuations in

excitation intensity, degradation and inhomogeneous distribution of the probe can

all lead to drift in the sensor response, thus limiting the lifetime and reproducibility

of devices. Self-compensating sensors contain an additional probe which independently monitors such changes, thus enabling correction of the sensor response

to the analyte. For example, luminescence-based oxygen sensors are known to

suffer from interference due to temperature fluctuations, whilst the response of

optical glucose sensors oscillates due to a fluctuating supply of oxygen to glucose


The optical response of many of the luminescence-based probes used in oxygen

and pH sensors is temperature dependent. The incorporation of a temperaturesensitive reference probe (e.g. MgOEP, Eu(III) diketonate) enables internal calibration of the sensor response. Ideally the temperature probe should be insensitive

to the analyte, but alternatively it can be immobilised in an analyte impermeable

polymer bead. The design of multi-probe luminescence-based optical sensors is

not trivial. Many probes exhibit very broad emission bands, meaning that the entire

visible spectrum can be effectively covered by just two probes. Consequently, in

the absence of both judicious selection of probes exhibiting the required analyte

response and an independently-resolvable spectral response, multi-probe sensors

are susceptible to considerable signal cross-talk and poor signal resolution. This

effect is further exacerbated when indicator probes with overlapping absorption

and emission bands are present at concentrations high enough to reach the critical

distance (*5 to 7 nm) for RET to occur.

In some cases it may be advantageous to amplify the optical response, to

improve the signal-to-noise ratio and therefore the sensor sensitivity. This may be

achieved by the addition of inorganic colloidal nanoparticles, e.g. TiO2, to the

sensor layer [10]. Since TiO2 particles scatter all visible wavelengths more or less

equally, their addition improves the interaction of light with the optical probe in

the sensor layer, effectively increasing the surface area exposed to the analyte.

Optimum scattering enhancement comes from colloidal particles whose diameter

are *0.5 times the probe emission wavelength. Alternatively, the signal may be


Optical Sensors and Probes


amplified using metal-enhanced fluorescence (MEF), where resonant coupling

between surface plasmons from metal nanoparticles or a nanostructured metal film

in close proximity to a lumophore probe can modify the radiative decay rate

leading to enhancement of the fluorescence signal [49].

12.4.4 Evaluation of Sensor Response

Some applications require that the analyte distribution over relatively large areas is

monitored in real time. This has been achieved by the development of sensor

paints. Sensor paints consist of an optical probe dispersed in a viscous polymer

solution, which can then be sprayed or painted onto the system of interest [50].

The sensor response is typically monitored using fluorescence imaging to obtain a

quantitative image of the analyte distribution. Sensor paints have found considerable application for the measurement of the air pressure gradient across the

surfaces of aeroplanes, spacecraft and racing cars in wind tunnels. As pressure

sensitive paints (PSPs) contain an oxygen-sensitive luminescent probe, they more

specifically measure the surface oxygen partial pressure gradient. Since many

oxygen-sensitive lumophores exhibit cross-sensitivity to temperature, a second

reference lumophore that corrects for excitation and temperature variations is often

added. These ideas have been extended to monitor pH and oxygen gradients in

marine environments and oxygen distribution across human skin [50].

Alternatively, it may be desirable to use a sensor which provides a quantitative

response to the analyte that is easily detected by the eye alone. This is particularly

useful for safety monitoring situations where a rapid indication of the analyte is

required. While colorimetric sensors may appear well suited to this task, the

detection limits of absorption-based devices are often inherently too low to be of

practical use. To overcome this limitation, a series of luminescence-based sensors

for the detection of oxygen using colour-change technology have been developed

[51]. The sensor is prepared by incorporating two (or more) lumophores with

different oxygen sensitivities and emission colours in a single device. In the

presence of oxygen, the emission from each lumophore is quenched at different

rates, resulting in a gradual ‘‘traffic-light’’ shift in the sensor emission colour

across the red-yellow-green spectral region with increasing oxygen concentration

(Fig. 12.7). This approach enables both rapid qualitative/semi-quantitative oxygen

detection and quantitative measurements with high sensitivity, depending on the

application requirements. However, since colour is a very subjective phenomenon,

the objective description of colour changes can be challenging. The CIE (Commission Internationale de l’Éclairage) system of colorimetry provides a numerical

description of colour, known as x,y colour coordinates, that is based on the sensitivity of the human eye to light across the visible spectral region (see Chap. 14).

CIE x,y coordinates provide a particularly convenient system to describe the

response of a luminescent colorimetric sensor when used in a qualitative or semiquantitative way, enabling calibration and prediction of the sensor response.


R. C. Evans and P. Douglas

Fig. 12.7 CIE xy colour coordinates (left) and the observed sensor emission colour (right) of a

dual-lumophore sensor at different % pO2. Revised from [51]

12.4.5 Multi-Analyte Sensing

The development of optical sensors that respond simultaneously and independently to different analytes is desirable for the analysis of complex samples. This

requires a multiplexed approach, whereby the spectral and/or time-dependent

optical response of the sensor must be resolved and unequivocally assigned to each

individual parameter simultaneously. The most straight-forward approach to

multiple analyte detection is to combine several analyte-specific sensors in a single

miniaturised sensor array. For laboratory-based analysis, microwell plates are a

cost-effective solution, with each well functionalised with a sensor specific to a

given analyte. For field sensing, fibre optic bundles, composed of thousands of

single core fibres bound together, but with each fibre maintaining its own independent light pathway, are promising. Each fibre in the bundle may be coated with

a different optical probe, thus enabling simultaneous detection of multiple analytes. However, the disadvantage of both these approaches is that the inherent

spatial distance between the sensors prevents the sample from being in contact

with more than one sensor at the same time, thereby preventing truly simultaneous

analyte detection.

In the truest sense, a multiplex sensor would use a single probe to detect each

analyte. However, in practice, finding a single material capable of meeting this

requirement is challenging. Multi-sensors (predominantly luminescence-based)

which contain several analyte-specific probes in a single device are therefore more

common [52]. However, determination of the correct probe combination is by no

means trivial and requires similar considerations to those mentioned previously for


Optical Sensors and Probes


internal calibration probes. Two sensor configurations are possible: (1) a singlelayer device in which multiple probes are homogeneously dispersed in a single

host layer, or (2) a multiple-layer arrangement, where several probe-host blends

for each analyte are immobilised as discrete layers. In single-layer sensors the

individual probes are often immobilised in permeation-selective polymer beads to

minimise signal cross-talk and to prevent analyte interference.

While dual-analyte luminescence-based sensors for a variety of parameters e.g.

O2/CO2, O2/temperature, O2/pH, O2/glucose are relatively common-place [52], a

triple-sensor for pH, O2 and temperature has only recently been realised [53]. The

device was comprised of a single sensor layer in which polymer beads containing

lumophores sensitive to either oxygen, temperature or pH were dispersed in a

polyurethane hydrogel. Judicious selection of the polymer-lumophore combination

enabled selective-permeation to the correct analyte. For example, the luminescence of the temperature-probe was considerably quenched by oxygen, but this

effect was minimised by incorporating it into poly(vinyl chloride), which has a low

oxygen permeability. Resolution of the response from each probe was achieved

either by using an optical band-pass filter or, since emission decay times of each

probe differed, gated (time-resolved) luminescence spectroscopy. It is interesting

to note that most multi-sensors contain an oxygen-sensing component, since

oxygen often acts as an interferent in sensors for other analytes. Moreover, oxygen

is also consumed or produced during the chemical or enzymatic reactions used to

detect CO2 and glucose, and consequently monitoring fluctuations in the oxygen

feedstock is essential for the reliable calibration of these sensors.

12.4.6 Optical Sensor Arrays

If several probes in an array respond to multiple analytes, but to a different extent,

this may be utilised to quantify several analytes simultaneously. Low cost miniaturised detectors such as CCD cameras allow high-sensitivity imaging of the

sensor array, enabling spectral changes (spectral shifts, intensity changes, shape

variations and temporal response) in the presence of each analyte to be monitored.

Image processing via pattern recognition then provides a qualitative and quantitative means of determining the analyte concentration. Frequently employed pattern recognition methods include principal component analysis (PCA), artificial

neural networks (ANN), and linear discriminant analysis (LDA) [54, 55].

These cross-reactive sensor arrays are often called optoelectronic noses (for

vapour or gas detection) or optoelectronic tongues (for detection in solution), since

they aim to mimic the mammalian olfactory and gustatory systems, respectively,

by producing a composite response that is unique to each analyte. Both luminescence-based and absorbance-based artificial noses and tongues have been

developed for a wide range of analytes including organic vapours, ligating organic

molecules and simple anions [54]. For example, an absorbance-based colorimetric

array capable of differentiating between 19 different toxic industrial chemicals has


R. C. Evans and P. Douglas

been developed [56]. In this array 36 different chemically-responsive dyes (e.g.

acid indicators, base indicators, vapochromic, solvatochromic) were mixed with an

ormosil host and printed as *1 mm diameter dots onto a polymer membrane. The

pattern of colour change across the array provided a unique molecular fingerprint

for the analyte mixture with detection limits of *2 to 500 ppm. A fluorescencebased cross-reactive array capable of identifying and quantifying metal cations in

soft drinks has also been described [57]. Each probe in the array contained the

same cation receptor (8-hydroxyquinoline) attached to a different conjugated

chromophore. Metal complexation resulted in change in the resulting metalloquinolinolate fluorescence (e.g. fluorescence enhancement, energy transfer, heavy

metal quenching) yielding a fingerprint-like pattern of responses for each sensorcation complex which could be discriminated by PCA and LDA.

12.5 Conclusions and Future Perspectives

In this chapter we aimed to demonstrate that while optical sensing is now

well-developed, there is still considerable scope for new research in this vibrant

and every-growing field. While the fundamental principles of optical sensing are

well-established, there is still the need for new probes with superior optical

properties, and also improved selectivity and sensitivity towards more unusual

analytes. There have been significant recent advances in both the design of sensor

platforms and strategies for improved device performance; however, there is still

more to be done. Device miniaturisation, critical for the success of wireless sensor

networks and remote sensing schemes, will continue to be a crucial area of growth

and developments in the key areas of nanotechnology, microfluidics and plasmonics are likely to drive future trends. Optical sensing will continue to play an

important role in real-time monitoring of environmental, health and security

parameters, and with a significant amount of research being conducted in this field,

long-range continuous monitoring across entire cities may soon become a reality.


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