Optical Waveguide Chemical Sensors - ACS Symposium Series (ACS

Chapter 24

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 20, 2018 | https://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch024

Optical Waveguide Chemical Sensors John F. Giuliani Chemistry Division, U.S. Naval Research Laboratory, Washington, DC 20375-5000

Optical waveguide chemical sensors have demonstrated their practical value not only for the detection of toxic vapors and gases in air, but also their adaptability to liquid phase detection of biomolecules. The basis for the success of our sensor program has been the strong multidisciplinary research effort into identifying and understanding the physical and chemical properties of organic materials including dyes and polymers, which form the reagent waveguide sensor coatings that give these devices their required sensitivity and specificity. In addition to the exploitation of cylindrical glass optical waveguide geometries, our work has widened to include the fabrication of multi-waveguide structures on glass substrates for the purpose of obtaining chemical sensor arrays on a single support. The last six years at NRL have seen the rapid development and diversification of optical waveguide chemical sensors for the detection of toxic gases and vapors in air, and small molecules in aqueous solutions. Our multidisciplinary approach has lead to the successful utilization of a large number of selective, chemically reversible and highly sensitive solid organic reagent film coatings. In addition to the exploitation of hollow cylindrical glass optical waveguide substrates, our work has widened to include the fabrication of integrated planar sensors. Although the development of practical optical sensors is our primary goal, both types of devices have been very useful in detecting subtle dynamic equilibrium surface interactions produced at the reagent film/waveguide glass interface. The basic principles and a general survey of chemical sensor applications based on fiber optics, have been adequately given by Seitz (1) and Angel (2). It is quite evident from the numerous This chapter not subject to U.S. copyright Published 1989 American Chemical Society Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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references c i t e d , that o p t i c a l sensors are rapidly taking t h e i r place as the sensor of choice for a v a r i e t y of chemical detection applications. I w i l l confine this report to o p t i c a l waveguide sensor development at our laboratory. The research e f f o r t to be presented discusses b r i e f l y some geometrical design considerations and the reasoning behind the p a r t i c u l a r geometry f i n a l l y chosen. Extensive vapor and solution phase detection data taken with our sensors w i l l be summarized. F i n a l l y some preliminary data r e l a t i n g to the f a b r i c a t i o n of planar structures and multi-element sensor "arrays" w i l l be presented. Waveguide Geometries One of the useful properties of o p t i c a l sensors i s that they can be formed into a v a r i e t y of geometrical shapes depending on t h e i r desired use. For example, Figure 1 displays three d i f f e r e n t useful geometric configurations. The s o l i d rod shape i s used generally i n the form of a drawn-out f i b e r . Early i n our work we decided on the h o l l o w - c y l i n d r i c a l shape based on ease of coating, mechanical strength, and the s i m p l i c i t y of coupling commercial LED ( l i g h t sources) and photodetectors. Moreover, i t has been shown by Kapany and P o n t a r e l l i , (3) that this geometry leads to high d i f f e r e n t i a l s e n s i t i v i t y f o r detecting small surface changes i n r e f r a c t i v e index. Also, as seen i n Figure 1, the number of o p t i c a l r e f l e c t i o n s per unit length for a thin-walled hollow cylinder can be several hundred for the 90 mmx 1.1 mmx 0.8 mm c a p i l l a r y tube. Most of our chemical sensor studies have been performed using t h i s hollowc y l i n d r i c a l geometry, i n which one end i s closed i n a l e n s - l i k e surface for focusing the multiply r e f l e c t e d l i g h t into a small commercial detector (4). We have, however, i n the l a s t few years explored the micro-planar geometry, both from the viewpoint of f a b r i c a t i o n and application to sensing vapors. More w i l l be said about t h i s l a t e r on i n this report. Hollow-Cvlindrical Waveguide Sensor Figure 2 shows a schematic of our reagent-coated sensor incorporated i n a gas chamber i n which the vapor or gas can be admitted and allowed to flow over the waveguide surface. Also shown i s the l i g h t source and c i r c u i t r y for an LED which i s coupled into one end of the cylinder, and a photodetector coupled to the other end, with i t s associated electronics. A f u l l description of t h i s device has been given as well as i t s application to a v a r i e t y of vapor detection situations (5,6,7,8). Application of the NRL Sensor to Selected Vapors Table I summarizes the application of the NRL sensor f o r the detection of four toxic vapors (Column 1), various s e l e c t i v e reagent coatings (Column 2), the lowest vapor concentration detected (Column 3), and the probable reaction mechanism (Column 4). The types of vapors besides ammonia, include an organophosphonate, and two s u l f u r compounds. The reagent coatings included a v a r i e t y of dyes, ( i . e . , oxazine, lead-phthalocyanine (Pb-Pc), lead tetraphenylprophrin, (Pb-

Murray et al.; Chemical Sensors and Microinstrumentation ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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366

Υ^/\/\/\/κ


ROD

Τ

HOLLOW CYLINDER

MICRO

3

PLANAR

LENGTH NUMBER

OF REFLECTIONS

Figure 1.

( Ν )

c x

THICKNESS

Various o p t i c a l waveguide geometries.

+ 5-15V

TTL

2 N 2 2 2 2

flow-thru

••P

l

n

c · 11

i

waveguide

Vo

photodiode 4*"

/

Selective

Coating

sample chamber

Figure 2. Schematic of the o p t i c a l , electronic, and flow chamber components f o r the NRL o p t i c a l waveguide sensor.



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TTP) , a Michlor's Ketone "EMKO", and an oxime dye incorporated with a c a t a l y s t . The polymer coatings tested were polyvinylpyrrolidone (PVP), poly-methylmethacrylate (PMM), polyepichlorohydrin (PEH), and polyethylene maleate (PEM). Note that with the exception of the oxazine dye, a l l other dyes show no color r e v e r s i b i l i t y . However, a l l the polymers show r e v e r s i b i l i t y to these vapors i n d i c a t i n g physical adsorption predominates the vapor/film interaction. The o p t i c a l coupling of the evanescent wave at the interface i s thought to be affected by solute-solvent, surface wetting, and film/glass adhesion interactions. More w i l l be said about t h i s e f f e c t l a t e r i n connection with s o l u b i l i t y measurements i n polymer films.

Table I. Optical Waveguide Detection of Toxic Vapors

Vapor

Coating Reagent

Lowest Concentration Detected,Δ

Ammonia

Oxazine Dye+

8 ppm

Dimethyl MethylPhosphonate

Pb-Pc Pb-TTP

8 ppm 20 ppm

*PVP+ *PMM+ *PEH+ *PEM+

5 ppm 25 ppm 10 ppm 20 ppm

•EMKOOxime/Catalyst

9 ppm 6 /ig/liter

Methane Sulfonyl Chloride Benzene Sulfonyl Chloride

Reaction Mechanism

Colorimetric Changes

Solubility Interactions

Colorimetric Changes Oxime/Catalys t

8 ppm

+ Reversible Charge Δ Dry A i r * Polymers

Solution Phase Small Molecule Detection Lubbers and Optiz (9) and Andrade et a l (10) have employed f i b e r optic sensors f o r the continuous measurement of chemical reactions i n b i o l o g i c a l systems. High s e n s i t i v i t y may be achieved using these sensors to measure fluorescent-tagged antigens or antibodies i n competitive binding immunoassay reactions i n solution. The NRL work involves the covalent attachment of a colorimetric redox dye indicator f i l m to the outer surface of the hollow c y l i n d r i c a l waveguide, which when immersed i n aqueous s o l u t i o n can r e v e r s i b l y detect various reducing species. The method of coating


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the c y l i n d r i c a l waveguide and experimental demonstration of the detection of redox agents has been described i n d e t a i l (11,12). Figure 3 shows a t y p i c a l dynamic reversible c o l o r i m e t r i c response when the viologen coated waveguide surface i s exposed a l t e r n a t e l y to d i s t i l l e d water/sodium d i t h i o n i t e (Na2S204) solutions, where Na2S204 i s a strong reducing species. Figure 4 displays the dynamic reversible colorimetric detection of the formate ion i n which a successful l i n k i n g of both the redoxviologen indicator dye and the formate dehydrogenase (FDH) enzyme to the waveguide surface leads to the reaction of the bound enzyme with the formate ion HCOO- i n the presence of a co-enzyme NAD. The production of NADH by the enzyme-catalyzed reaction r e s u l t s i n a reduction o f the p o s i t i v e charge on the bound viologen which i s detected by a change i n color ( i . e . , from c o l o r l e s s to blue). Our hollow c y l i n d r i c a l waveguide structure has also been coated with a human h-IgG antibody enzyme and used to detect a fluorescent labeled antigen kg*/Oye i n a immunoassay-type reaction. A l l of the surface phase reactions detected by the hollow c y l i n d r i c a l o p t i c a l waveguide sensor are summarized i n Table I I , which l i k e Table I shows four columns depicting the detected analyte, the s e l e c t i v e waveguide coating, the lowest concentration detected, and the reaction mechanism. Fabrication of Integrated O p t i c a l Waveguide Sensors We have s u c c e s s f u l l y photopolymerized two commercial oligomers by means of lasers and incoherent u l t r a - v i o l e t l i g h t sources using the technique of photolithography. Our procedure has been described i n (13. 14). The polymerized waveguide channel i s a commercial

Table I I .

NRL Optical Waveguide Solution Phase Studies

Analyte

Coating

Concentration

Reaction

Ag*/Dye Labeled

h-Ig G

66 nMoL/L

Antigen-Antibody ComplexFluorescence

HC02H (Formic Acid)

Viologen/FDH

7.5 mMol/L

Enzyme Catalyzed Charge Transfer-

HC02Na (Sodium Formate)

Viologen/FDH

2 nMol/L

Colorimetric

Na2S204

Viologen

>0.005 mg/ml

Charge Transfer

H2 gas/water

Viologen

1 μΜοΙ/L

NADH/H20

Viologen

0.5 g/L

Absorption DependentFluorescence



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Figure 3. Typical dynamic response of the viologen-coated waveguide exposed alternately to d i s t i l l e d water/sodium d i t h i o n i t e solutions.


369


370


SCAN TIME

(SECONDS)

Figure 4. Colorimetric detection of the formate ion f o r the dual f i l m coated waveguide.



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thiolene compound which has the photo-initiator incorporated i n the s t a r t i n g material. Fiber optic " p i g t a i l s " are attached to the i n i t i a l l y unpolymerized material (14). Figure 5 displays the o p t i c a l response of this detector to acetone vapors at concentrations of 10,000 ppm, probed by a neon incoherent l i g h t source. Table III l i s t s a series of saturated vapors produced at 22°C to which this sensor was exposed. As can be inferred from the table, s o l u b i l i t y apparently plays an important role i n the s e l e c t i v e o p t i c a l detection. A more d e t a i l e d investigation of the s o l u b i l i t y properties of polymers and t h e i r o p t i c a l response to selected vapors w i l l be discussed i n the next section.

Table I I I . Optical Response of the Photopolymerized Thiolene S t r i p Shaped Waveguide Structure on a Glass Substrate to Various Vapors i n a Nitrogen Carrier Flow System Vapors

Optical Response

2-Propanol

No

Toluene

No

Acetone+

Yes

Water

No

Ammonium Hydroxide

No

Chloroform*

Yes

Xylene

No

Benzene No Source: Reprinted from r e f . 17. •Solvent for the monomer

Vapor/Polvmer

S o l u b i l i t y Interactions

As was mentioned i n the l a s t section, the s o l u b i l i t y of the polymerized planar sensor material f o r a s p e c i f i c vapor appeared to be important. In this connection we have c a r r i e d out a series of experiments i n which two polymers having known s o l u b i l i t y parameters (£) have been deposited on the hollow c y l i n d r i c a l waveguide structure as t h i n films produced by dipping the waveguide into the polymer/solvent solution. Rapid evaporation of the solvent r e s u l t s i n a t h i n f i l m (15). The polymers investigated, were polyethylene maleate (PEM) and polyfluoropolyol (PFP) whose indices of r e f r a c t i o n are 1.484 and 1.413, respectively. These films were exposed separately to a series of vapors having a broad range of s o l u b i l i t y


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0.25 « 0

1

1

10

20

1

1

30 40 S C A N TIME (tec)

1

1

1

50

60

70

Figure 5. Relative dynamic response of the thiolene waveguide sensor to saturated acetone vapors. (Reprinted from r e f . 17.)



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parameters (6) between 8.6 (cal/cm3)l/2 and 23.4 (cal/cm3)l/2. The vapor concentrations were on the order of 10,000 ppm. Figures 6 and 7 show the measured change i n o p t i c a l transmission as a function o f the s o l u b i l i t y parameters f o r the various vapors. Note that there i s a s i g n i f i c a n t dip i n transmission f o r both polymer films when the s o l u b i l i t y o f the vapor matches that of the polymer within several percent. The two compounds DMMP (dimethyl methylphosphonate) and DMAC (N, N-dimethylacetamide) have s o l u b i l i t y parameters which l i e within a few percent of the two polymer films tested. The o p t i c a l changes detected are believed to be caused by the softening of the bulk polymer f i l m which i n turn produces a s i g n i f i c a n t change i n i t s light-guiding properties. Our dynamic response data show that i n a l l cases the surface phenomena i s reversible when the o p t i c a l signal i s monitored p r i o r to and a f t e r the vapor i s removed from the dynamic equilibrium flow system. Multi-Element Optical Waveguide Sensor Arrays A single chemical sensor with a single coating would not be a p r a c t i c a l chemical detector for a p a r t i c u l a r gas or vapor dispersed i n a mixture o f interferents. Hence, multiple sensors, and a few s e l e c t i v e reagent coatings would be desirable, so that discrimination between classes of compounds may be obtained using pattern recognition techniques (16). Our i n i t i a l array of sensors made use of the previously characterized h o l l o w - c y l i n d r i c a l structure already discussed (4). Figure 8 outlines two general methodologies ( i . e . , single vs multiple photodetectors) coupled to eight single c y l i n d r i c a l waveguides. In t h i s report we w i l l describe experiments using a single photo-diode detector and an array of eight color emitting LED sources attached to eight separate hollow c y l i n d r i c a l waveguides. The i n i t i a l experiments to be described, involve the detection of pH and redox reactions i n aqueous solutions, using two dye coatings ( i . e . a l i z a r i n yellow and the previously discussed viologen redox i n d i c a t o r ) . A block diagram of the multi-element system i s shown i n Figure 9. For t h i s configuration, a small computer can monitor and control normal system operations. When the computer i s not d i r e c t l y monitoring the control interface control board (CCIB), i t can execute system control routines. As can be seen i n Figure 9, the CCIB i s an interface which has d i r e c t influence over a l l input/output operations, while i t s control parameters are monitored by a 6502 based microcomputer (Commodore 64). The main functions therefore, of the computer are those of parameter control and data manipulation. The CCIB i s a free-running hardwired c o n t r o l l e r complete with i t s own clock for running the multi-element system i n a r e a l time mode. I t interrogates the channels on a variable time basis. The data channels are monitored and updated within the computer and passed to the CCIB v i a d i g i t a l outputs. The system software senses when the CCIB i s making the amplified photo-detector s i g n a l available f o r further processing and storage. For t h i s system, the minimum on-times f o r the LED output s t a b i l i z a t i o n are about 1 to 2 milliseconds. For a t y p i c a l data-cycle a single LED i s activated and a f t e r a suitable delay, the amplified photo-detector



374

C H E M I C A L SENSORS A N D

MICROINSTRUMENTATION

100

6

8

10

12

14

16

18

S O L V E N T SOLUBILITY P A R A M E T E R S o

20 s

22

24

(cal/cm ) 3

1/2

Figure 6. Optical response of PEM coated c y l i n d r i c a l waveguide to a series of vapors encompassing a range of s o l u b i l i t y parameters. The names of some of the vapors tested, are shown i n the figure. (Reprinted from r e f . 15.)



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100 WATER METHANOL

99 DMMP

DMAC

98 • • • P F P FILM D A T A

δ = 11.Of 97 TMIN - 9 9 . 1 %

2

t «CAL

96

S

P/ MON M

Σ

G

95

94

93

h

I

I

I I I I I I I I 10

12

14

16

18

I I I I I I 20

24

22

S O L V E N T SOLUBILITY P A R A M E T E R S ô ( c a l / c m ) s

3

1/2

Figure 7. Optical responses of PFP coated c y l i n d r i c a l waveguide exposed to the same range of vapors and s o l u b i l i t y parameter range as shown i n Figure 6 · (Reprinted from r e f . 15.)


376


SINGLE

DETECTOR

>-

Couplers

Optical Waveguide Chemical Sensors - ACS Symposium Series (ACS

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