Luận án tiến sĩ: New generation chemical sensors and sensor systems

Contents Abstract 1.3.1 Sol-Gel Derived Sensor Materials that Yield Linear Calibration Plots, High Sensitivity, and Long Term 1.3.2 Tailored Xerogel-Based Sensor Arrays and Artificial N

NEW GENERATION CHEMICAL SENSORS AND SENSOR SYSTEMS

in partial fulfillment of the requirements for the

degree of

Doctor of Philosophy

UMI Number: 3244233

3244233 2007

Copyright 2007 by Tehan, Elizabeth Christine

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by ProQuest Information and Learning Company

In memory of my beloved mother, Mrs Bertha (Kawam) Tehan

I have been blessed by God with my talents and the ability to share them with people around me Named or otherwise, these people have invaluably helped me Though words cannot express my thanks and appreciation to them, the impact they have had and continue to have on my life will never be overlooked

I would like to express my sincere appreciation to my advisor, Professor Frank V Bright Your leadership and wisdom have helped shape me both professionally and personally Moreover, I thank you for your patience and understanding, despite the times

I felt undeserving You are a great Teacher and Individual

I have been afforded the opportunity to work with many exceptional

professionals This includes my Ph.D committee: Professors Luís A Colón, Michael R Detty and Troy D Wood This also includes Drs Alexander N Cartwright and Albert H Titus Your insight and guidance as a group and individually, have helped me greatly and

I have deep respect for each of you as professionals In addition, special thanks to Dr Brian MacCraith, for allowing me the memorable experience of working with the NCSR group at Dublin City University I would like to acknowledge others too numerous to mention, including past and present Bright Group Members Much of this work may not have been done without the help of: Drs Eun Jeong Cho, Ying Tang, Zunyu Tao,

Michael Davenport, Vamsy Chodavarapu, and Ms Rachel Bukowski, Mr William Holthoff and the exceptionally skilled machinists in the UB Arts & Sciences Machine Shop Thank you all for your collaborations and friendships

Some people whose life’s path crosses our own forever impact us For me, one

compliment me immeasurably and for your love and companionship You and your family have been so welcoming and supportive of me; thank you I look forward to our continued love and friendship

My entire extended family and friends have always supported me, especially through this endeavor, and I am so fortunate to be among them I regret that some, who are dear to my heart, are not here to share in this accomplishment

My immediate family has been outstanding I am so proud of and proud to be a

part of them My parents have supported my education in every way and I am eternally grateful for that They have always shown me love and encouragement Mom, you have taught me so much though your life and your memory Dad, you have been my

inspiration and motivation Thank you to my sister, Victoria who, with her husband, Timothy and children, Andrew and Ryan have extended their family to include me I appreciate our time together and the way you have opened your home to me Thank you

to my brother, Louis and his wife Wendy, though we are not close in distance, I

appreciate your confidence in me and motivation to excel in everything I do Thank you

to my brother, Joseph I appreciate your heartening outlook on life and the way you share your humor with me, always making me smile I love each of you dearly and thank you for your unending love and encouragement

Finally, each project discussed in this document was made possible by financial support from various agencies and are gratefully acknowledged These agencies include: the National Science Foundation, the Gerald A Sterbutzel Fund at UB, the John R Oishei Foundation, the Office of Naval Research, and the Interdisciplinary Research and Creative Activity Fund of the State University of New York at Buffalo

Contents

Abstract

1.3.1 Sol-Gel Derived Sensor Materials that Yield Linear

Calibration Plots, High Sensitivity, and Long Term

1.3.2 Tailored Xerogel-Based Sensor Arrays and Artificial

Neural Networks Yield Improved O2 Detection Accuracy

1.3.3 Creating a Diverse Sensor Response from a Single Sensor

1.3.4 Chemical Sensing Systems Using Xerogel-Based Sensor

1.3.5 Tailored Quartz-Based Pins for High-Density Microsensor

Chapter 3 Sol-Gel Derived Sensor Materials That Yield Linear Calibration

Plots, High Sensitivity and Long-Term Stability 31

3.3.2 Preparation of [Ru(dpp)3]2+- doped Octyl-triEOS/TEOS

3.4.4 Time-Resolved Intensity Decays 50

Chapter 4 Tailored Xerogel-Based Sensor Arrays and Artificial Neural

Networks Yield Improved O2 Detection Accuracy and Precision 64

4.3.3 Luminophore-doped Sol Solution Preparation 70

4.4.1 Instrumentation for Characterizing the Arrays 72

4.6.2 Artificial Neural Networks to Improve Overall Sensor

4.6.3 Performance of Sensor Elements After Contact with Rat

4.7 Conclusions 93

Chapter 5 Creating Diversified Response Profiles from a Single

Quenchometric Sensor Elements by Using Phase-Resolved

Chapter 6 Chemical Sensing Systems Using Xerogel-Based Sensor

6.2.1 Luminescence- Based Quenching: Recognition and

6.6.2 Comparison of Xerogel-Based Sensor Systems Using

6.6.3 Comparison of Other Analytical Figures of Merit 146

7.2.2 Substrates 153 7.2.3 Preparation of Pin Silanization Solutions 153 7.2.4 Preparation of Luminophore-Doped Sol 153

List of Figures

Figure 1.1 A simplified schematic of a chemical sensor system (A) A

photonic system (B) An enlarged view of a sensor element

Figure 2.1 A description of the sol-gel process (A) The simplest sol-gel

process for a tetraalkoxysilane (B) The sol-gel process for a

Figure 2.2 A simplified Jablonski diagram describing luminescence

Figure 2.3 (A) Photograph of the Cartesian Technology model MicroSys

5100 array printing system, showing a microwell plate platform,

a washing station, pins in the pin mount and substrate platforms (B) Pins loaded in the pin mount for printing 24

Figure 2.4 (A) Photograph of a quill pin and (B) mechanism for printing with

this type of pin (C) Photograph of a 200 µm solid pin and (D) its

Figure 3.1 SEM images of Octyl-triEOS/TEOS composite xerogel films

three months) 40

Figure 3.2 SEM images of the varios regions of the 80% Octyl-triEOS/

20% TEOS xerogel film showing phase separation and

Figure 3.3 Effects of aging time and xerogel composition on the O2

Figure 3.4 Typical Stern-Volmer plots for [Ru(dpp)3]2+-doped

Octyl-triEOS/TEOS xerogels that have aged for three months

The solid lines represent the best fit to a Demas model (TEOS)

Figure 3.5 Effects of xerogel composition on the average Stern-Volmer

quenching constant for three month old samples 52

Figure 3.6 Typical excited-state luminescence intensity decay traces for

[Ru(dpp)3]2+-doped Octyl-triEOS/TEOS xerogel composites in

an N2 environment Xerogels have been aged for three months (A) Pure TEOS (B) 20% Octyl-triEOS / 80% TEOS (C) 40% Octyl-triEOS / 60% TEOS.(D) 50% Octyl-triEOS / 50% TEOS

Figure 3.7 Effects of xerogel composition on the average [Ru(dpp)3]2+

excited-state fluorescence lifetime and the bimolecular

Figure 4.1 Chemical structures of the precursors and lumiophores used in

Figure 4.2 O2-dependent false color images from an array of O2 responsive

xerogel- based sensor elements based on co-doping [Ru(bpy)3]2+ and [Ru(dpp)3]2+ within C8-TEOS/TEOS xerogels See Table 4.1 for the compositions of sensor elements labeled 1-5 75

Figure 4.3 Typical intensity-based Stern-Volmer plots for the sensor

elements shown in Figure 4.1 The lines that pass through the data represent the best fit to Eq 1 (1 and 5) or Eq 2 (2-4) 77

Figure 4.4 Simplified schematic of a single pore within the

C8-TEOS/TEOS class II xerogel showing the envisaged distribution of [Ru(bpy)3]2+ and [Ru(dpp)3]2+ molecules 82

Figure 4.5 Illustration of the (A) Forward and (B) Backward propagation

Figure 4.6 Typical intensity-based Stern-Volmer plots for O2 responsive

sensors before and after being subjected to rat plasma and rat whole blood The lines that pass through the data represent the best fit to Eq 1 (before) or Eq 5 (after) The recovered

parameters that describe the response profiles are compiled in

Figure 5.1 Traditional approaches that have been used to create a continuum

of response profiles from a chemical sensor Three hypothetical sensor elements are shown under each approach which would yield three different response profiles to a particular target

Figure 5.2 Frequency-domain luminescence schematics (A) Phase-

modulation concept Excitation (ex), emission (em), and the luminescence phase shift (θ) are shown (B) The phase resolution experiment with the detector phase angle (θDx) set at four different values (θD1 to θD4x) The shaded region denotes the area under the modulated emission that is integrated by the π gate (C) The resulting phase-sensitive luminescence intensity (PSLI) that results from the different θDx settings in (B) 104

Figure 5.3 Simplified phase-sensitive instrument schematic The

modulation frequency (f) is controlled by the function generator,

the detector phase angle (θD) is adjusted by the lock-in amplifier, and the sample composition that reaches the sensor element is

Figure 5.4 Simulated (A, B) and experimental (C, D) O2-dependent, phase

sensitive Stern-Volmer plots for the [Ru(dpp)3]2+-doped

octyl-triEOS/TEOS-based xerogels at f = 20 kHz In the simulations

τ0 = 5 0 µs and K SV = 0.1 O2%-1 (B, D) θD = 90o data omitted 111

Figure 5.5 Simulated (A, B) and experimental (C, D) O2-dependent, phase

sensitive Stern-Volmer plots for the [Ru(dpp)3]2+-doped

octyl-triEOS/TEOS-based xerogels at f = 50 kHz In the simulations

τ0 = 5 0 µs and K SV = 0.1 O2%-1 (B, D) θD = 67.5o data omitted 113

Figure 6.1 The 40-pin DIP package for CMOS detectors 127

Figure 6.2 (A) Photograph of the photodetector array The top three rows

are phototransistors and bottom three rows are photogates (B) Circuit diagram of phototransistor active pixel sensor (APS)

Figure 6.3 I-V relationship for the APS circuit 132

Figure 6.4 Photograph of a lateral p-n-p phototransistor 134

Figure 6.5 (A) Block diagram of the setup The sensor/detector system can

be stand-alone, but the data analysis component is also shown to highlight the testing and evaluation of the sensor system (B) Diagram of the LED light source, sensor element film, optical filters, sample flow chamber, and CMOS-based chip detector 137

Figure 6.6 Photograph of sensor sample chamber (flow cell holder with

inlet and exhaust), mounting apparatus, optical filter, CMOS

Figure 6.7 (A) Stern-Volmer plot and (B) Modified Stern-Vomer plot for

Figure 7.1 Digital photographs of the five pin types evaluated in this

research (The scale is different in each image; the tip

Figure 7.2 The quartz pin system (A) Pin holder schematic (B) Pin

holder photograph (C) Quartz pin silanization/cleaning

Figure 7.3 Cycling protocol for cleaning, silanizing, and stripping the

Figure 7.4 False color fluorescence images of Rhodamine 6G-doped

C8-TEOS/TEOS-based xerogels printed with 600, 400 and

200 µm diameter solid tungsten (A-C, respectively), 75 µm stainless steel quill (D), and 12 µm fused silica pins (virgin) (E) (F) Expansion of one element within (E) 164

Figure 7.5 False color fluorescence images for Rhodamine 6G-doped pin

printed xerogels (A) Clean quartz pin and C8-TEOS/TEOS

(B) C8-silanized quartz pin and C8-TEOS/TEOS (C) Pin in (B) after HF treatment (recleaned) (D) Metal or ceramic pin and C8-TEOS (E) APTES-silanized quartz pin and C8-TEOS

(F) C8-silanized quartz pin and C8-TEOS (G) Pin in A after 50,000 prints (H) Pin in B after 50,000 prints 167

Figure 7.6 Intensity-based Stern-Volmer plot from a 4x4 array of 9 µm

diameter, O2-responsive [Ru(dpp)3]2+-doped C8-TEOS/TEOS xerogels (Insets) Raw false color images in the absence and

List of Tables

Table 3.1 Effects of composition and aging time on the O2 quenching

of [Ru(dpp)3]2+- doped Octyl-triEOS/TEOS xerogels 49

Table 3.2 Effects of Octyl-triEOS/TEOS xerogel composition on the

recovered [Ru(dpp)3]2+ time-resolved decay kinetics 55

Table 4.1 Luminophore-doped sol solution compositions used to prepare

the xerogel-based sensor elements used in this research 71

Table 4.2 Recovered parameters from the Stern-Volmer plots in Figure

4.3 under different fitting constraints 79

Table 4.3 Recovered concentrations from synthetic unknown O2 samples 87

Table 4.4 Recovered O2 quenching parameters before and after the

sensors were subjected to rat plasma or whole blood 91

Table 6.1 Head-to-head comparison of the CMOS-based detectors with

Table 7.1 Effects of pin type and surface chemistry on the final feature

List of Acronyms and Symbols

<KSV> Average Stern-Volmer quenching constant

<τ> Average excited state lifetime

AMI Alternate mark inversion

ANN Artificial neural network

CMOS Complimentary metal-oxide semiconductor

GPIB General purpose interface bus

I Luminescence intensity in the presence of quencher

I0 Luminescence intensity in the absence of quencher

KSOM Kohonen self-organizing map

KSV Stern-Volmer quenching constant

laser Light amplification by stimulated emission of radiation LED Light emitting diode

M0 Demodulation in the absence of quencher

MLP Multilayer perception

MOSIS Metal oxide semiconductor implementation service

octyl-triEOS n-octyltriethoxysilane

ORMOSIL Organically modified silane

PPCSA Pin printed chemical sensor array

PSLI Phase sensitive luminescence intensity

PSLI0 Phase sensitive luminescence intensity in the absence of quencher

[Ru(dpp)3]2+ tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II)

Ru(dpp)3Cl2 tris(4,7’-diphenyl-1,10’-phenanthroline) ruthenium(II) chloride

pentahydrate

SEM Scanning electron microscope

τ excited state lifetime in the absence of quencher

τ0 excited state lifetime in the presence of quencher

Chemical sensors have become a valuable method for the detection and

quantification of a multitude of analytes The ideal sensor and sensor system would be portable, inexpensive, readily fabricated, and monitor and quantify several key analytes both simultaneously and reversibly

The dissertation focuses on the development of new generation photonics-based chemical sensors and sensor systems by exploiting the benefits from sol-gel processing technology, pin printing methods, and low-power detection schemes There are three main topics within this work First, we describe techniques to provide a diverse sensor response to an analyte Secondly, we describe the development of a compact sensor system Lastly, we improve multianlayte sensing capabilities through advancements in materials and array manipulation Each Chapter is designed to be independent and may

be read in any order

Chapter 1 Introduction

1.1 Sensors and Sensor Arrays

There has been a tremendous push to develop a low power and portable sensor system for the detection of multiple analytes.1-3 This detection system should allow simultaneous, stable and accurate detection of multiple analytes These features are important particularly when such sensors are used in areas where modifications to the system (e.g changing a battery) are not often convenient The growing trend in

measuring “everything” in a sample has lead to the development and improvement of array-based sensor systems.4-6 An array based sensor is ideal due to the ability of such arrays to simultaneously detect and a variety target analytes In particular, pin-printed chemical sensor arrays allow for a large number of potential analytes detected

simultaneously while still being compact.2, 7 In addition to creating a small-sized array, researchers have been able to decrease the size of optical sensor systems even further by using a light emitting diode (LED) or radioluminescent light source instead of a laser as

an excitation source.8-11

Sensors typically consist of a recognition element that is immobilized onto a platform and selectively interacts with an analyte of interest (target analyte) When the recognition element binds or associates with the target analyte, the result of this

interaction produces a detectable change in analytical signal.12 Figure 1.1 illustrates a generic, photonically-based sensor system (A), the key components within the sensor elements (B), and the response profiles (C) In general, the analytical signal may be electrochemical, thermal, mass or optical Regardless of the signaling method, a sensor should exhibit a reversible and reproducible response

Figure 1.1 A simplified schematic of a chemical sensor system (A) A photonic

system (B) An enlarged view of the sensor element (C) The responses

Target Analyte

Platform Luminophore

Time

C

Target Analyte

Platform Luminophore

Platform Luminophore

Time

C

Time

C

In an optically-based sensor the following components are required: a light

source, an analyte-responsive sensing layer, a readout system and power Useful chemical sensors require the chemical recognition element be immobilized at or within some form

of transducer head This can be accomplished by either physisorption, covalent

attachment, or entrapment.12 To avoid some of the problems associated with

physisorption and covalent attachement, entrapment has become an attractive means of immobilizing the recognition element.13-15 Sequestration eliminates most of the problems associated with other methods, however leaching can persist.16 Sol-gel science and

technology14, 17-19 provides a pathway to avoid many of these problems and allow one to sequester chemical agents, typically without any chemical modification to the

immobilized species Sol-gel derived materials with different physical or physiochemical properties, such as pore size, shape and surface area or polarity, refractive index, and density can be easily formed depending on the precursors used and sol-gel processing conditions

Under certain conditions a silica sol can be spin-cast or dip-coated onto a surface

to create a thin layer xerogel Under other conditions, particles or fibers can be fabricated Thin films have been frequently used to prepare chemical sensors because of their

relatively short path lengths for diffusion, which improve sensor response times and recovery rates Bulk xerogel monoliths have been used in spectroscopic investigations20-

for chemical modification25, 26 Sol-gel derived materials are also optically transparent, making them ideal for the development of chemical and biochemical sensors that rely on changes in an absorbance or luminescence signal27 These materials are also thermally stable

1.2 Sensors and Instrument Design

Many scientists are interested in detecting several analytes in a complex sample simultaneously.28-30 An ideal multianalyte sensor array would be portable, inexpensive, readily fabricated, and monitor and quantify several key analytes both simultaneously and reversibly Sol-gel processing techniques allow one to form micrometer-size sensing platforms, such as printed features or thin films, each doped with different recognition or sensing chemistries.1, 31 By using a charge coupled device (CCD) for detection, more than one analyte can be quantified simultaneously Small, low-power light sources have been investigated for miniaturization of the sensor head and the entire imaging system LEDs are relatively inexpensive light sources with stable power output and a long lifetime.32-35These small light sources, coupled with micrometer-scale sensing platforms, appropriate sensing chemistries and small array detectors should lead to the development of portable, simple, low-cost, versatile, chemical and biochemical sensing schemes.1, 8, 9, 36-38 Despite advancements in sensor technology, there continues to be many challenges For example, there are needs for efficient, portable, and robust devices for field or home/consumer deployment that possess the analytical figures of merit (detection limits, dynamic range, response time, and sensitivity) of standard laboratory systems

1.3 Research Goal and Dissertation Scope

The general goal of the research reported in this dissertation is to develop new generation chemical sensors and sensor systems that can be used for off-site multianalyte determination The focus of this work aims to: 1) generate new sensor array devices that provide a diverse sensor response for the target analyte 2) create a compact sensor system that can be used in remote areas, with few power restrictions 3) improve

multianalyte sensing capabilities through advances in materials and array manipulation

This thesis is divided into eight Chapters with each Chapter self-contained so that they may be read in any order Chapter 2 will provide additional, general background information not covered in the subsequent Chapters The remaining chapters are divided

as described below Chapter 8 will conclude the work reported and suggest future

directions

1.3.1 Sol–Gel-Derived Sensor Materials that Yield Linear Calibration Plots, High

Sensitivity, and Long Term Stability

Chapter 3 describes novel O2 sensing materials based on spicoated

n-octyltriethoxysilane (Octyl-triEOS) / tetraethylorthosilane (TEOS) composite xerogel films These sensors are based on the O2 quenching of tris(4,7-diphenyl-1,10-

phenanthroline)ruthenium(II) ([Ru(dpp)3]2+) sequestered within the xerogels Scanning electron microscopy and luminescence measurements (steady-state and time-resolved) have been used to investigate the structure of these films, their analytical figures of merit, and determine the underlying reasons for their observed performance The results show that certain [Ru(dpp)3]2+-doped Octyl-triEOS/TEOS composites form uniform, crack-free

calibration curves and excellent long-term stability For example, an 11 month old sensor based on 50 mol % Octyl-triEOS exhibits more than four-fold greater sensitivity in

comparison to an equivalent sensor based on pure TEOS Over an 11 month time period, the sensitivity of a pure TEOS-based sensor drops by more than 400% whereas a sensor based on 50 mol % Octyl-triEOS remains stable (RSD = 4%)

1.3.2 Tailored Xerogel-Based Sensor Arrays and Artificial Neural Networks Yield

Improved O 2 Detection Accuracy and Precision

The objective of the research described in Chapter 4 is to develop arrays of tuned chemical sensors wherein each sensor element responds to a particular target analyte in a unique manner By creating sol-gel-derived xerogels that are co-doped with two

luminophores at a range of molar ratios, one can form suites of sensor elements that can exhibit a continuum of response profiles An artificial neural network (ANN) was trained

to “learn” to identify the optical outputs from these xerogel-based sensor arrays By using the ANN in concert with our tailored sensor arrays a 5-10 fold improvement in accuracy and precision for quantifying O2 in unknown samples was observed The response

characteristics of these types of sensor elements after contact with rat plasma/blood was observed Contact with plasma/blood caused ~15% of the luminophore molecules within the xerogels to become non-responsive to O2 This behavior is consistent with rat albumin blocking certain pore sub-populations within the mesoporous xerogel matrix thereby limiting O2 access to the luminophores

1.3.3 Creating a Diverse Sensor Response from a Single Sensor Element Using

Phase Fluorimetry

The sol-gel process is a useful method for creating chemical and biochemical sensors There has been extensive research done on modifying the xerogel precursors to enhance and diversify sensor performance.39,40, 41 A drawback with these types of sensors

is that there is only one set of detection parameters, and thus, one response profile from each individual sensor Diversified response for a particular analyte have successfully been created by creating an array of sensors that exhibit unique response profiles to the same target analyte.39, 42 This approach requires multiple sensing elements Chapter 5 demonstrates a new strategy wherein phase-resolved fluorescence techniques are

exploited to create a diversified sensor response from a single xerogel-based O2 sensor element Simple phase fluorimetric techniques are described and by adjusting the

excitation modulation frequency and detector phase angle, different analytical response characteristics for the same sensor film are observed This approach provides a means to generate regions of ultra high and low sensitivity from the exact same sensor element This Chapter compares simulated data with experimental results showing the creation of unique response profiles from a single sensor element (i.e., diversified response) The operating principle behind the O2 sensors is the quenching of [Ru(dpp)3]2+ sequestered within a nanoporous xerogel The luminescence and O2- responsiveness of this molecule has been studied extensively and is used as a model sensor system.43, 44

1.3.4 Chemical Sensing Systems Using Xerogel-Based Sensor Elements and CMOS

Photodetectors

Chapter 6 presents the first example of an integrated complementary metal-oxide semiconductor (CMOS) photodetector coupled with a solid-state xerogel-based thin-film

different CMOS-based detector systems to results obtained by using a standard

photomultiplier tube (PMT) or CCD detector Because the chemical sensor elements are governed by the Stern–Volmer relationship, the Stern–Volmer quenching constant is used

as the primary comparator between the different detectors All of the systems yielded Stern–Volmer constants from 0.042 to 0.049 O2%-1 The results show that the CMOS detector system yields analytical data that are comparable to the CCD- and PMT-based systems The disparity between the data obtained from each detector is primarily

associated with the difference in how the signals are obtained by each detector as they presently exist Reversibility in the operation of the sensor system has also been

observed The CMOS-based system exhibits a response time that is faster than the

chemical sensor element’s intrinsic response time, making the CMOS suitable for dependent measurements The CMOS array detector also uses less than 0.1% the

time-electrical power in comparison to a standard PMT or CCD The combined

xerogel/CMOS system represents an important step toward the development of a

portable, efficient sensor system

1.3.5 Tailored Quartz-Based Pins for High-Density Microsensor Array

Fabrication

There have been many advances in the development of chemical sensor arrays with simultaneous multianalyte sensing capabilities The limiting factor in the number of simultaneous analytes that can be detected or the number of sensing features within a given detection area, is the sensing feature size This size is directly related to the sensor applicator or in the case of pin printed chemical sensor arrays, the pin Chapter 7

describes a new pin that allows us to print features in an on-demand manner on the order

of 10 µm in diameter: ten times smaller than existing methods This capability yields a hundred fold increase in the number of analytes one can detect This Chapter compares all existing pin printing methods and the analytical figures of merit for sensor arrays created with this new approach

1.4 References

1 Cho, E J.; Tao, Z.; Tehan, E C.; Bright, F V Analytical Chemistry 2002, 74,

6177-6184

2 Cho, E J.; Bright, F V Analytical Chemistry 2002, 74, 1462-1466

3 Rowe, C A.; Tender, L M.; Feldstein, M J.; Golden, J P.; Scruggs, S B.;

MacCraith, B D.; Cras, J J.; Ligler, F S Analytical Chemistry 1999, 71,

7 Tam, J M.; Song, L.; Walt, D R Talanta 2005, 67, 498-502

8 Cho, E J.; Bright, F V Analytical Chemistry 2001, 73, 3289-3293

9 Cho, E J.; Bright, F V Analytica Chimica Acta 2002, 470, 101-110

10 Holthoff, W G.; Tehan, E C.; Bukowski, R M.; Kent, N.; MacCraith, B D.;

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11 Watkins, A N.; Wenner, B R.; Jordan, J D.; Xu, W.; Demas, J N.; Bright, F V

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12 Janata, J Principles of Chemical Sensors; Plenum Press: New York, NY, 1989

13 Sommerdijk, N A J M.; Wright, J D Journal of Sol-Gel Science and

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14 Ingersoll, C M.; Bright, F V Chemtech 1997, 27, 26-31

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of Sol-Gel Processing; Academic Press: New York, 1990

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20 Zheng, L.; Reid, W R.; Brennan, J D Chemistry of Materials 1999, 11, 12

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McGarvey, J J Journal of Materials Chemistry 1997, 7, 1473-1479

Chapter 2 Theory

2.1 Immobilization through the Sol-Gel Process

A useful method to immobilize the sensing chemistry is based on the sol-gel process.1, 2 This process allows us to create solid-state platforms for the chemical sensors that are optically transparent which are both chemically and thermally stable Figure 2.1A illustrates the simplest sol-gel process for a tetraalkoxysilane (e.g., tetramethoxysilane, (TMOS) or tetraethoxysilane (TEOS)) This process consists of three main steps:

hydrolysis, condensation and polycondensation In the hydrolysis step, a metal or metal alkoxide is hydrolyzed in the presence of an acid or base catalyst to form a silanol and the corresponding alcohol In the condensation step, either two hydroxyl groups, or a hydrolyzed alkoxide and a hydroxyl group condense to form sol particles and water or the corresponding alcohol In the final polycondensation step, the sol particles can further condense to form an optically transparent, three-dimensional xerogel network.3 In

semi-preparation of the sensors discussed in this dissertation, all sol processing is conducted under acid catalysis to form the three-dimensional gel network This approach provides ideal platforms for optically based sensors

ORMOSILs (organically modified silanes) can be added to tetraalkoxysilanes to create Class II ORMOSILs and tailor the xerogel to the specific needs and applications for the sensor being designed.4-8 Class II ORMOSILs contain an R’ group attached to the

Si atom that cannot be hydrolyzed Figure 2.1B illustrates the sol-gel process for a Class

II ORMOSIL, showing the R’ group remaining within the final xerogel Because of this feature, Class II ORMOSILs are useful when tailoring a xerogel for a specific sensor platform4, 9-12

Figure 2.1 A description of the sol-gel process (A) The simplest sol-gel process for a

tetraalkoxysilane (B) The sol-gel process for a Class II ORMOSIL