Chemical Sensors and Microinstrumentation - ACS Publications

Chapter 2

New Anion- and Gas-Selective Potentiometric Sensors 1

Downloaded by NORTH CAROLINA STATE UNIV on December 30, 2017 | http://pubs.acs.org Publication Date: August 24, 1989 | doi: 10.1021/bk-1989-0403.ch002

M. E. Meyerhoff, D. M. Pranitis , Η. S. Yim, N. A. Chaniotakis, and S. B. Park Department of Chemistry, University of Michigan, Ann Arbor, MI 48109

Although highly selective membrane electrodes are now used routinely for direct measurements of various cations (e.g.Η ,Κ , Na , Ca ), there are r e l a t i v e l y few analogous sensing devices that can be u t i l i z e d to determine specific anions and gases. In this paper, recent progress in the design of simple anion and gas selective potentiometric chemical sensors based on solvent/polymeric membranes w i l l be summarized. In the case of anion sensing, the incorporation of various metal-ligand complexes, including metalloporphyrins, within polymeric membranes has resulted in the development of highly selective sensors for sulfite, salicylate and thiocyanate. The sulfite selective membrane may be further used i n conjunction with an outer gas permeable membrane to design SO selective sensing systems. Existing cation and the newer anion polymeric membrane electrodes can also be employed to fabricate novel differential gas sensing cells that exhibit enhanced sensitivity toward the analyte gas. The principles of this differential approach are demonstrated via the design of a new ammonia selective sensor. +

+

+

2+

2

Advances i n t h e development o f ion-selective e l e c t r o d e s o v e r t h e p a s t 20 y e a r s have f o c u s e d p r i m a r i l y on t h e d e s i g n o f membrane-based s e n s o r s f o r m o n i t o r i n g Current address: Lever Brothers Company, Edgewater, NJ 07020

0097-^156^9/0403-0026$06.00/0 © 1989 A m e r i c a n C h e m i c a l Society

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

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Potentiometric Sensors


physiologically important cations. Perhaps t h e most useful potentiometric ion-selective devices a r e now f a b r i c a t e d w i t h s o l v e n t / p o l y m e r i c membranes. Selectivity o f t h e s e e l e c t r o d e s f o r one i o n o v e r o t h e r s i s d i c t a t e d by the r e l a t i v e i n t e r a c t i o n s o f ions i n t h e sample s o l u t i o n w i t h a c t i v e c o m p o n e n t s o f t h e o r g a n i c , p o l y m e r i c membrane phase. In p r a c t i c e , t h e s e l e c t i v i t y of polymer membrane e l e c t r o d e s c a n be e v a l u a t e d u s i n g t h e w e l l known N i c o l s k y equation :

a l l

= Κ +

(0.059/z ) l o g

(1)

i

where a i i s t h e a c t i v i t y o f t h e a n a l y t e i o n w i t h z i ; a j i s t h e a c t i v i t y o f a n interfèrent i o n w i t h pot ζ., k,, and

E

.

i s the potentiometric

selectivity

charge charge

coefficient,

n i s expressed i n v o l t s . Either n e u t r a l or charged c a r r i e r molecules (e.g. crown e t h e r s , n a t u r a l a n t i b i o t i c s , s y n t h e t i c ionophores, e t c . ) c a n be doped i n t o p o l y m e r i c membranes t o a c h i e v e desired selectivities. However, d e s p i t e s u c c e s s e s i n the f a b r i c a t i o n o f s e l e c t i v e a n d s t a b l e c a t i o n e l e c t r o d e s (15) , t h e d e v e l o p m e n t o f a n a l o g o u s d e v i c e s f o r a n i o n s h a s thus f a r been limited by the i n a b i l i t y to identify appropriate a n i o n - s e l e c t i v e ionophores. c

e

In t h i s paper, r e s u l t s a r e presented from recent s t u d i e s regarding t h e use o f m e t a l - l i g a n d complexes as a n i o n s e l e c t i v e membrane c o m p o n e n t s . This anion sensor work i s one component o f a l o n g e r t e r m e f f o r t t o d e v e l o p new a n d i m p r o v e d p o t e n t i o m e t r i c g a s s e n s o r s . Indeed, Figure 1 illustrates how a p p r o p r i a t e polymeric i o n s e l e c t i v e membranes c a n be u s e d i n e i t h e r s t a t i c (A) o r flow-through g a s s e n s i n g c o n f i g u r a t i o n s ( B ) . By s e n s i n g i o n i c forms o f t h e gases i n a r e c i p i e n t b u f f e r (rather t h a n pH c h a n g e s i n a n o n - b u f f e r e d l a y e r as i n c o n v e n t i o n a l Severinghaus s t y l e gas sensors), enhanced s e l e c t i v i t y over other acidic a n d b a s i c g a s e s c a n be a c h i e v e d . This concept has been s u c c e s s f u l l y a p p l i e d f o r t h e s e l e c t i v e d e t e c t i o n o f ammonia i n s o l u t i o n a s w e l l a s i n a i r (6-8) using an i n t e r n a l nonactin-based polymeric membrane s e n s i t i v e t o ammonium i o n s . Extending t h i s concept t o o t h e r g a s e s , p a r t i c u l a r l y SO2, NO2, a n d CO2, r e q u i r e s t h e development of suitable anion selective polymeric membranes. While i t i s most advantageous t o u s e membrane e l e c t r o d e s s e n s i t i v e t o i o n i c forms o f t h e a n a l y t e gases, c e r t a i n f a b r i c a t i o n advantages a r e a l s o r e a l i z e d merely by



28

CHEMICAL

SENSORS

AND MICROINSTRUMENTATION

. Polymer Membrane

A) p Ë I o r h S e t e c t i v e Electrode?

Aqueous |H ONH

^NHJ*OH"

Ι

I H 0*2N0 ^ N O i • |H OSO ^HSO^H*

I

2

3

2

2

2

2

C0

2

NH

3

N0 S0 2

2

Gm-permeatte

Gas or Liquid

Membrane

Sample

ico.

z

H 0 2

NHjiNÔgSC^ 1

ni C0

^HCOi+H

2

H 0+NH 2

2

^NH;>OH"

3

2

2

to Polymer Membrane ISE ,

1

+

H O+2NO ^NO£+NO +2H* H OS0 ^HSOi+H* 2

• Récipient Solution

3

/

Figure 1. S c h e m a t i c o f s t a t i c (A) a n d c o n t i n u o u s f l o w (Β) g a s s e n s i n g system using solvent/polymeric i o n s e l e c t i v e membrane e l e c t r o d e s a s d e t e c t o r s .


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MEYERHOFF ET AL.


29


u s i n g p o l y m e r r a t h e r t h a n g l a s s pH s e l e c t i v e membranes a s internal transducers i n more c l a s s i c a l Severinghaus designs ( 9 , 10) . Moreover, as d e s c r i b e d below, t h e combination o f u s i n g these two d i s t i n c t l y d i f f e r e n t gas d e t e c t i o n schemes ( g a s - i o n d e t e c t i o n a n d pH detection) simultaneously i n a single differential gas sensor configuration can o f f e r a unique enhancement i n gas sensitivity.

Anion S e l e c t i v e Membrane Electrodes The l i t e r a t u r e i s f u l l o f r e p o r t s t h a t d e s c r i b e t h e response properties o f anion s e l e c t i v e solvent/polymeric membrane e l e c t r o d e s ( 4 , 5, 1 1 , 1 2 ) . In almost every instance, the active membrane components of such electrodes are lipophilic quaternary ammonium or phosphonium s a l t s . Since the organic cations of these s a l t s are very hydrophobic, the p o s i t i v e l y charged s i t e s r e m a i n i n t h e o r g a n i c membrane p h a s e . Assuming no d i r e c t i n t e r a c t i o n between t h e l i p o p h i l i c c a t i o n i c s i t e s and counter anions i n t h e membrane p h a s e , p h a s e boundary p o t e n t i a l s a r e g e n e r a t e d a t t h e sample/membrane i n t e r f a c e via the partitioning o f t h e more h y d r o p h i l i c counter anions i n t o t h e aqueous sample. Thus, f o r a f i x e d anion a c t i v i t y i n t h e sample, t h e s o l u b i l i t y o f t h e g i v e n anion i n t h e o r g a n i c membrane p h a s e w i l l d i c t a t e t h e m a g n i t u d e of t h e phase boundary p o t e n t i a l generated. With regard t o selectivity, t h i s mechanism always r e s u l t s i n t h e s o c a l l e d H o f m e i s t e r p a t t e r n (C10 ~" > SCN~ > i " > Br"* > C l " > 4

HC03~) r e g a r d l e s s o f t h e s t r u c t u r e o f q u a t e r n a r y ammonium o r p h o s p h o n i u m s a l t i n c o r p o r a t e d i n t h e membrane. In p h y s i c a l terms, t h e Hofmeister p a t t e r n c o r r e l a t e s d i r e c t l y with the free energies of hydration f o r the various anions. To d e v i a t e f r o m H o f m e i s t e r behavior, the polymeric membrane m u s t b e d o p e d w i t h s p e c i e s t h a t d i r e c t l y i n t e r a c t with, or p r e f e r e n t i a l l y solvate, selected anions. To date, such d e v i a t i o n s have been o b s e r v e d i n o n l y a few instances. F o r example, carbonate anion s e l e c t i v i t y can be induced by incorporating trifluoroacetophenone d e r i v a t i v e s a s t h e membrane a c t i v e c o m p o n e n t (13) . In this case, the carbonate ion i s a strong enough n u c l e o p h i l e t o react w i t h t h e e l e c t r o p h i l i c carbonyl group of the ketone, forming a s t a b l e dianion adduct i n t h e membrane p h a s e . Organotin compounds have been shown t o exhibit selectivity toward thiocyanate, c h l o r i d e , and phosphate, depending on t h e s t r u c t u r e o f t h e s p e c i e s doped into t h e membrane (14-16). Membranes c o n t a i n i n g a lipophilic derivative of vitamin B12 have exhibited unusually strong response t o n i t r i t e and thiocyanate, apparently a r i s i n g from p r e f e r e n t i a l c o o r d i n a t i o n o f these anions with the cobalt ( I I I )center of the corine s t r u c t u r e (17) .



30

CHEMICAL SENSORS AND

MICROINSTRUMENTATION

Thiocyanate and S a l i c y l a t e S e l e c t i v e Membranes Using Metalloporphyrins as Ionophores. Studies with metalloporphyrins have suggested t h a t such s p e c i e s can a l s o y i e l d a n i o n s e n s i t i v e membranes w i t h anti-Hofmeister selectivity sequences (18, 1 9 ) . Potentiometric anion responses of the membranes d o p e d w i t h metalloporphyrins may b e a t t r i b u t e d t o t h e d i r e c t i n t e r a c t i o n o f s a m p l e anions with a x i a l coordination s i t e s of the c e n t r a l metal. Thus, controlling the e q u i l i b r i u m constant f o r anion coordination should influence the potentiometric anion selectivity sequences observed. Variations i n the coordination affinities of anions may b e a c h i e v e d by a l t e r i n g the c e n t r a l metal i o n and/or changing s t r u c t u r a l appendages on t h e p o r p h i n e r i n g . To d e m o n s t r a t e t h e s e p r i n c i p l e s , t h e p o t e n t i o m e t r i c anion s e l e c t i v i t i e s o f membrane e l e c t r o d e s p r e p a r e d by doping the various metalloporphyrins shown i n F i g u r e 2 into plasticized PVC membranes (at approx. 1 wt % o f porphyrin) have been examined. Table I summarizes t h e r e s u l t s o f such s t u d i e s . pot T a b l e I. P o t e n t i o m e t r i c S e l e c t i v i t y C o e f f i c i e n t s , l o g k o f D i f f e r e n t M e t a l l o p o r p h y r i n - PVC M e m b r a n e s ' Relative to Chloride c l

x

3

Anion CI" Br" I" C10 IO4" SCN~ Sal" 4

a

b

b

TDMA-C1 M n [ T P P ] C 1 M n [ T P P P ] C l (21 (1L (21 0.0 0.6 2.8 4.6 4.0 3.0 2.7

0.0 0.3 1.5 1.5 1.8 1.3 2.1

Evaluated i n 0.05 a c i d ) b u f f e r , p H 5.5 Sal" = salicylate

0.0 1.0 2.3 2.8 2.5 5.2 3.5 M

Mn[TBrNP]CI (41 0.0 0.2 1.6 0.4 0.6 3.5 2.0

Sn[TPP]Cl2 (11 0.0 0.1 0.1 0.4 0.2 1.3 3.8

MES(2-Morpholinoethanesulfonic

For comparison purposes, selectivity data i s also p r e s e n t e d f o r membranes p r e p a r e d w i t h a t y p i c a l quaternary ammonium e x c h a n g e r , tridodecylmethy1ammonium chloride (TDMA-C1) (D . A l lthe porphyrin species tested yield s e l e c t i v i t i e s which c l e a r l y deviate from t h a t observed w i t h TDMA-C1. M o r e i m p o r t a n t l y , t w o compounds, Μη[TPPP]CI (2.) a n d S n [ T P P ] C I 2 (5.) , s h o w e x t r a o r d i n a r y selectivity toward t h i o c y a n a t e and s a l i c y l a t e , r e s p e c t i v e l y . In t h e case of Mn[TPPP]CI, adding conjugated and bulky s u b s t i t u e n t s i n t h e form o f twelve phenyl groups t o t h e basic tetraphenylporphyrin [TPP] s t r u c t u r e d r a m a t i c a l l y enhances t h e response and s e l e c t i v i t y toward thiocyanate



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Figure 2. S t r u c t u r e s o f some c o m p o u n d s incorporated i n t o p o l y ( v i n y l c h l o r i d e ) membranes f o r t h e d e v e l o p m e n t of anion sensors; ( J J t r i d o d e c y l m e t h y l ammonium c h l o r i d e (TDMA-C1); (2J chloro (5,10,15,20 tetraphenylporphyrinato) manganese ( I I I ) ( M n [ T P P ] C l ) ; Q) chloro (5,10,15,20) -tetra(triphenyl) phenylporphyrinato) m a n g a n e s e ( I I I ) (Mn [ T P P P ] C l ) ; (±) a, a, a, a - c h l o r o (5, 10, 15,20) tetrakis ( 8-bromo-1-naphthy1) porphyrinato) m a g a n e s e ( I I I ) ( M n [ T B r N P ] C I ) ; (5) d i c h l o r o (5,10,15,20 tetraphenyl-porphyrinato) tin (IV) (Sn[TPP]Cl ). 2


32

CHEMICAL SENSORS AND


( r e l a t i v e t o Mn[TPP]Cl (2J ) . In f a c t , the resulting membrane e l e c t r o d e r e s p o n d s i n a N e r n s t i a n fashion to t h i o c y a n a t e i n t h e r a n g e f r o m 0.01 mM t o 10 mM (see F i g u r e 3) . The h i g h s e l e c t i v i t y o v e r c h l o r i d e and salicylate


m a k e t h i s s e n s o r p o t e n t i a l l y u s e f u l f o r d e t e c t i n g SCN in b i o l o g i c a l samples (e.g., u r i n e and s a l i v a ) where e l e v a t e d levels of thiocyanate c o r r e l a t e with excessive c i g a r e t t e s m o k i n g (20) . The d a t a o b t a i n e d f o r t h e S n [ T P P ] C l 2 ~ b a s e d membranes c l e a r l y i l l u s t r a t e how a c h a n g e i n t h e c e n t r a l m e t a l i o n of the porphyrin can influence potentiometric anion selectivities. As shown i n T a b l e I , i n c o r p o r a t i o n o f t h i s compound i n t o membranes y i e l d s a s e l e c t i v i t y p a t t e r n w h i c h i s m a r k e d l y d i f f e r e n t t h a n t h a t f o r Mn[TPP]Cl. Salicylate (o-hydroxybenzoate) remains the p r e f e r r e d anion, but the r e l a t i v e response toward t h i s ion over other anions is enhanced significantly. The increased response to s a l i c y l a t e i s due t o i t s s t r o n g e r i n t e r a c t i o n as an a x i a l l i g a n d , p r o b a b l y due t o t h e o x o p h i l i c n a t u r e o f t h e S n ( I V ) center. Since the c h a r g e on the t i n center is +4, i n t e r a c t i o n of anion ligands at e i t h e r a x i a l coordination s i t e mandates t h a t t h i s m e t a l l o p o r p h y r i n f u n c t i o n as a c h a r g e d c a r r i e r t y p e i o n o p h o r e i n t h e membrane p h a s e . As shown i n Figure 4, the selectivity of the S n [ T P P ] C l 2 - b a s e d membrane o v e r o t h e r aromatic organic compounds i s q u i t e h i g h . This fact coupled with the membrane's h i g h s e l e c t i v i t y o v e r c h l o r i d e (see T a b l e I) makes t h e r e s u l t i n g membrane e l e c t r o d e p o t e n t i a l l y u s e f u l for monitoring l e v e l s of free s a l i c y l a t e i n p h y s i o l o g i c a l samples. In t h i s regard, p r e l i m i n a r y r e s u l t s have been o b t a i n e d f o r t h e d e t e r m i n a t i o n o f s a l i c y l a t e i n s p i k e d and u n s p i k e d human u r i n e s a m p l e s ( T a b l e I I ) . As c a n be s e e n , there i s reasonably good c o r r e l a t i o n between the values obtained with the new salicylate selective membrane e l e c t r o d e and the c o n v e n t i o n a l c o l o r i m e t r i c method now used i n h o s p i t a l s . Unfortunately, at t h i s point, i n t e r p r e t i n g r e s u l t s f o r the e l e c t r o d e measurement of salicylate in blood samples i s c o m p l i c a t e d by t h e f a c t t h a t a l a r g e f r a c t i o n o f t h e t o t a l s a l i c y l a t e i s b o u n d t o p r o t e i n s ( 2 2 , 23) . The S n ( T P P ) C l 2 - b a s e d membrane e l e c t r o d e d e t e c t s "free" s a l i c y l a t e while the conventional c o l o r i m e t r i c procedure (i.e. the T r i n d e r method (21) or v a r i a t i o n s thereof) measures t o t a l s a l i c y l a t e c o n c e n t r a t i o n ( f r e e p l u s bound). The former i s the p h y s i o l o g i c a l l y a c t i v e form of the c o m p o u n d (24) . Thus, the s a l i c y l a t e s e l e c t i v e e l e c t r o d e c o u l d p r o v i d e a new a n a l y t i c a l t o o l f o r s c i e n t i s t s who are e x a m i n i n g t h e p h a r m a c o l o g i c a l e f f e c t s o f a s p i r i n and other




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33

34

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

Anion

Log

k^ s

0.0


Salicylate ο c-o"

Ο

-1.4

Benzoate

3-Hydroxy benzoate

l

Ç)- -°-

HO>

c-o-

/x

1

' ·

1

1.0

4-Hydroxybenzoate

OH H O - ^ ^ — c-o-

2,4-Dihydroxy benzoate

«1,4

2,6-Dihydroxybenzoate

^.6

2,5-Dihydroxybenzoate (Gentisate)

.t 3

OH

HO

O

H

OH OH

ο

ο

λ—c—NHCH - C - O"

Salicylurate

-1.5

2

ο CH -C-O-

a

.^8

Phenylacetate

2

ο - Q - ° - CH -C-O" 2

2,4-DIchlorophenoxyacetate

e 1

«j

(2,4-D)

ο H C--^^--S-O3

p-Toluenesulfonate

-2J

ο

ο Phenylphosphonate

.2.9

Figure 4. P o t e n t i o m e t r i c anion s e l e c t i v i t y c o e f f i c i e n t s , r e l a t i v e t o s a l i c y l a t e , o f Sn[TPP] CI2-PVC membrane t o w a r d v a r i o u s a r o m a t i c a n i o n s p e c i e s .


2.


MEYERHOFF ET A L

Table

II.

Comparison o f R e s u l t s O b t a i n e d f o r Measurement of S a l i c y l a t e Concentrations i n S p i k e d and N o n - S p i k e d Human U r i n e Samples by Sn ( T P P ) C I 2 B a s e d Membrane E l e c t r o d e a n d Conventional C o l o r i m e t r i c Method Sample


35

1A IB 1C 2A 2B 2C 3A 3B 3C 4A 4B 4C 5A 5B 5C

3

5

Electrode* ' (mM) 0,.32 1,.05 1,.79 0,.24 0..84 2,.02 0,.26 0,.86 2,.02 0,.18 0,.68 1,.53 0,.24 0,.78 1,.78

c

Colorimetric (mM) 0,.35 1,.02 1,.97 0..07 0,.88 2,.04 0..22 0,.80 1..90 0,.29 1,.02 1,.68 0..36 1,.02 1,.97

a

A-samples are unspiked; B-samples a r e Α-samples s p i k e d w i t h s a l i c y l a t e t o a c h i e v e a change i n c o n c e n t r a t i o n o f 0.7 9 mM; C-samples a r e Α-samples s p i k e d t o a c h i e v e change i n c o n c e n t r a t i o n o f 1.9 mM s a l i c y l a t e . ^average o f 3 determinations samples diluted 1:10 i n MES, ρH 5. 5 buffer for measurements. c

salicylates i n l i v i n g subjects. T h i s a p p l i c a t i o n cannot be pursued at this t i m e due t o t h e e l e c t r o d e s pH response. As shown i n F i g u r e 5, when t h e pH o f t h e sample i s i n t h e p h y s i o l o g i c a l range, d e t e c t i o n l i m i t s toward s a l i c y l a t e a r e poor. O n l y a f t e r t h e sample i s d i l u t e d i n low ρ H b u f f e r (e.g., pH 5.5) c a n s u b - m i l l i m o l a r l e v e l s o f s a l i c y l a t e be q u a n t i t a t e d a c c u r a t e l y . U n f o r t u n a t e l y , such a change i n pH i s l i k e l y t o d i s t u r b t h e s a l i c y l a t e - p r o t e i n binding equilibria. R e c e n t s t u d i e s have s u g g e s t e d that t h e t o t a l w a t e r c o n t e n t o f t h e membrane p l a y s an i m p o r t a n t r o l e i n t h e o b s e r v e d p o t e n t i o m e t r i c pH r e s p o n s e (18,25) . Thus, b y c h a n g i n g t h e p l a s t i c i z e r a n d / o r b y s w i t c h i n g t o p o l y m e r m a t r i c e s o t h e r t h a n PVC, a s i g n i f i c a n t r e d u c t i o n i n pH r e s p o n s e o f t h e membrane may be p o s s i b l e . 1

Sulfite/Bisulfite Selective Electrode B a s e d Q_n B J S ( D i e t h y l d i t h i Q c a r f r a m a t Q Merçyry (II) . The i n c o r p o r a t i o n o f m e t a l - l i g a n d c o m p l e x e s w i t h i n p o l y m e r i c membranes f o r t h e d e v e l o p m e n t o f new a n i o n s e n s o r s i s n o t l i m i t e d t o t h e use o f m e t a l l o p o r p h y r i n s . Indeed, r e c e n t s t u d i e s w i t h t h e Hg ( I I ) complex o f d i e t h y l d i t h i o c a r b a m a t e (Hg(DDC)2) have y i e l d e d a new membrane e l e c t r o d e w i t h r e l a t i v e l y high



36

CHEMICAL SENSORS AND MICROINSTRUMENTATION

s p e c i f i c i t y f o r s u l f i t e i o n s ( P r a n i t i s , D.M.; Meyerhoff, M.E. Anal. Chim. A c t a , i n press). The membrane i s p r e p a r e d b y d o p i n g t h e m e r c u r y complex a t 1.8 wt% i n a P V C matrix p l a s t i c i z e d w i t h dibutylphthalate. Potentiometric r e s p o n s e o f t h e membrane t o s t e p c h a n g e s i n s u l f i t e i o n a c t i v i t y i n p h o s p h a t e b u f f e r , pH 10.0, o c c u r i n < 2 m i n . When t h e e q u i l i b r i u m p o t e n t i a l s are plotted vs. the l o g a r i t h m o f s u l f i t e a c t i v i t y i n t h e sample s o l u t i o n , a calibration c u r v e s u c h a s t h a t shown i n F i g u r e 6 i s obtained. Near N e r n s t i a n b e h a v i o r i s o b s e r v e d i n t h e a c t i v i t y r a n g e o f 0.1 t o 1000 μΜ s u l f i t e . S e l e c t i v i t y o f t h e Hg ( D D C ) 2 ~ b a s e d membrane t o w a r d s u l f i t e r e l a t i v e t o o t h e r a n i o n s c o r r e l a t e s w i t h what one might expect f o r anionic species i n t e r a c t i n g with the Hg ( I I ) c e n t e r o f t h e c o m p l e x . As shown i n T a b l e I I I , i o d i d e , bromide, t h i o s u l f a t e , and t h i o c y a n a t e a r e major interferents. However, t h e membrane e x h i b i t s l i t t l e o r no response t o a wide range o f o t h e r anions including sulfate, phosphate, c h l o r i d e , and n i t r a t e , making i t potentially useful as an analyt i c a l sensor f o r measurements i n c e r t a i n samples, e . g . , i n m o n i t o r i n g added s u l f i t e l e v e l s i n f o o d and b e v e r a g e s .

Table

III.

Potentiometric

Selectivity

Hg(DDC)2 -Based Membrane

Coefficients

of

3

pot

Anion sulfite c h l o r i d e , s u l f o n a t e (MES), a c e t a t e , nitrite, citrate, nitrate, p e r c h l o r a t e , cyanate, s a l i c y l a t e , sulfate, bicarbonate/carbonate

l o g k 2SQ

0 < -4

bromide

0

thiocyanate

0

thiosulfate

0.5

iodide

j

7

S e l e c t i v i t y c o e f f i c i e n t s measured w i t h m a t c h e d - p o t e n t i a l , s e p a r a t e - s o l u t i o n method i n 10 mM p h o s p h a t e b u f f e r , pH 10.0. The mechanism by w h i c h t h e Hg(DDC)2 compound a c t s a s an ionophore for sulfite i s not yet c l e a r . Initial s t u d i e s i n d i c a t e that t h e mercury(II) species i s e s s e n t i a l f o r s u l f i t e response. When Zn (DDC)2 i s u s e d i n p l a c e o f


MEYERHOFF ET AL.

37



2.

-8

-6

-4

-2

Log (sulfite activity),M Figure 6. T y p i c a l c a l i b r a t i o n c u r v e Hg ( DDC ) 2 - P V C membrane electrode theroetical Nernstian slope l i n e ) .

toward s u l f i t e (solid line


for i s


38

C H E M I C A L SENSORS AND MICROINSTRUMENTATION

t h e m e r c u r y c o m p l e x , no s u l f i t e s e n s i t i v i t y i s o b s e r v e d . H o w e v e r , when d i p h e n y l m e r c u r y ( I I ) i s incorporated into polymeric membranes, s i g n i f i c a n t sulfite response i s observed although i t i s inferior to that found with Hg(DDC)2· I t appears that s u l f i t e interacts with the Hg(II) b y d i s p l a c i n g a t h i o c a r b o n y l s u l f u r o f t h e DDC l i g a n d , y i e l d i n g a n e g a t i v e l y charged d i v a l e n t complex. If so, i n c o n t r a s t t o t h e Sn(TPP)CI2 - s a l i c y l a t e system a b o v e , t h e Hg ( D D C ) 2 s p e c i e s s h o u l d b e c l a s s i f i e d a s a n e u t r a l c a r r i e r type ionophore. New P o t e n t i o m e t r i c

Gas S e n s i n g

Systems

A s s t a t e d p r e v i o u s l y , t h e d e v e l o p m e n t o f new anion s e l e c t i v e membrane e l e c t r o d e s c a n a l s o l e a d t o t h e d e s i g n of improved potentiometric gas sensing systems. At p r e s e n t , a l m o s t a l l c o m m e r c i a l gas s e n s o r s r e l y on t h e u s e o f g l a s s m e m b r a n e pH e l e c t r o d e t o d e t e c t p H c h a n g e s i n a thin film of electrolyte sandwiched between t h e g l a s s membrane a n d a n o u t e r g a s p e r m e a b l e membrane (typically microporous Teflon). This i s the s o - c a l l e d Severinghaus d e s i g n (2 6 ) . F o r e x a m p l e , a n S O 2 s e n s o r may b e f a b r i c a t e d by u s i n g an i n t e r n a l e l e c t r o l y t e c o n s i s t i n g o f sodium bisulfite a n d s o d i u m c h l o r i d e (27) . Equilibration of d i s s o l v e d SO2 i n t o t h e t h i n l a y e r o f e l e c t r o l y t e results in a pH change logarithmically proportional to the concentration of SO2 i n t h e sample. However, t h e s e l e c t i v i t y o f such a sensor i s l i m i t e d . In this case, other v o l a t i l e a c i d i c species, such as a c e t i c a c i d , N0 , a n d e v e n CO2 w i l l cause p o s i t i v e i n t e r f e r e n c e w i t h t h e m e a s u r e m e n t o f S O 2 (28) . X

S u l f u r D i o x i d e S e n s i n g B a s e d on S u l f i t e S e l e c t i v e Membrane Electrode. T h e Hg ( D C C ) 2 - b a s e d s u l f i t e s e l e c t i v e e l e c t r o d e d e s c r i b e d a b o v e c a n b e u s e d a s a t r a n s d u c e r t o d e v i s e new SO2 selective gas sensing systems according to the d e t e c t i o n schemes d e p i c t e d i n F i g u r e 1 . The a d d i t i o n o f t h e o u t e r g a s p e r m e a b l e membrane e n a b l e s m e a s u r e m e n t s t o b e made i n t h e p r e s e n c e o f i o n s w h i c h w o u l d n o r m a l l y b e m a j o r interfèrents t o t h e membrane e l e c t r o d e ( e . g . I , B r , S2O3"" etc.). Initial studies have focused on incorporating the s u l f i t e selective electrode i n a flowthrough gas sensing arrangement such as t h e one illustrated i n Figure 7. I t has been found t h a t t h e sulfite electrode e x h i b i t s a mixed response t o both s u l f i t e a n d b i s u l f i t e , w h i c h a l l o w s f o r t h e u s e o f a pH 6.0 b u f f e r e d r e c i p i e n t s t r e a m . Upon a c i d i f y i n g s a m p l e s c o n t a i n i n g s u l f i t e / b i s u l f i t e , s u l f u r d i o x i d e gas i s formed in proportion to the concentration of the sulfites present. T h i s g a s d i f f u s e s t h r o u g h t h e membrane o f t h e d i a l y z e r where i t i s t r a p p e d as a b i s u l t e / s u l f i t e mixture in t h e r e c i p i e n t b u f f e r stream. Potentiometric response of t h e downstream sulfite selective electrode i s 2


2. MEYERHOFF ET AL.


proportional concentration

Potentiometrk Sensors

39

to the logarithm of i n the original acidified

sulfite/bisulfite sample.

This approach g r e a t l y enhances t h e s e l e c t i v i t y f o r m e a s u r i n g s u l f i t e s a s SO2 r e l a t i v e t o s e n s i n g s y s t e m s t h a t detect SO2 v i a p H c h a n g e s i n a r e c i p i e n t e l e c t r o l y t e solution. I n d e e d , a s i l l u s t r a t e d i n F i g u r e 8, s u l f i t e c a n b e d e t e c t e d t o l e v e l s b e l o w 10 μ Μ e v e n i n t h e p r e s e n c e o f 0.1M a c e t i c a c i d . While t h e a c e t i c a c i d does permeate t h e gas membrane i n t o t h e r e c i p i e n t s t r e a m , a c e t a t e ions f o r m e d i n t h e r e c i p i e n t s t r e a m go u n d e t e c t e d by t h e Hg(DCC)2-based s u l f i t e s e l e c t i v e s e n s o r . This type of enhanced s e l e c t i v i t y w o u l d be e x t r e m e l y valuable when determining s u l f i t e s i n wine-vinegar o r other samples t h a t contain v o l a t i l e acidic species. Gas Sensing Membrane Electrode-Based E n h a n c e d Gas S e n s i t i v i t y . One frequent complaint r e g a r d i n g t h e use o f p o t e n t i o m e t r i c gas and i o n s e l e c t i v e membrane e l e c t r o d e s f o r a n a l y t i c a l p u r p o s e s i s lack o f p r e c i s i o n owing t o t h e l o g a r i t h m i c response of such devices. Thus, u n c e r t a i n t y i n measured p o t e n t i a l s o f -lmV w i l l r e s u l t i n ±4% p r e c i s i o n f o r s e n s o r s w i t h s l o p e s of 59 mV/decade a n d ^ 8 % f o r t h o s e devices based on r e s p o n s e t o d i v a l e n t i o n s ( e . g . , t h e a b o v e s u l f i t e a n d SO2 gas sensors) . One n o v e l approach suggested i n the l i t e r a t u r e f o r enhancing response slopes o f potentiometric sensors i s t o u s e s e v e r a l membrane e l e c t r o d e c e l l s i n series ( 2 8 , 29) . This arrangement r e s u l t s i n response s l o p e s η t i m e s t h e N e r n s t i a n v a l u e , where η i s t h e number o f two e l e c t r o d e c e l l s (working and r e f e r e n c e ) i n s e r i e s . Unfortunately, t h i s a p p r o a c h r e s u l t s i n a more c o m p l e x system where t h e number of electrodes required i s increased, as i s t h e number of separate sample compartments.

Differential Detectors w i t h

Recently, a novel two electrode differential p o t e n t i o m e t r i c c e l l f o r enzyme e l e c t r o d e s y s t e m s h a s b e e n d e s c r i b e d t h a t provides enhanced s u b s t r a t e sensitivities c o m p a r e d t o c o n v e n t i o n a l c e l l s composed o f a s i n g l e enzyme electrode and reference ( C h a , G.S.; Meyerhoff, M.E. Electroanalysis, i n press). The c e l l e m p l o y s two w o r k i n g enzyme e l e c t r o d e s , one w h i c h r e s p o n d s t o t h e a n a l y t e i n the p o s i t i v e p o t e n t i a l d i r e c t i o n v i a detection of cations, and o n e w h i c h r e s p o n d s t o t h e same a n a l y t e b u t i n a negative d i r e c t i o n owing t o anion d e t e c t i o n . A similar a p p r o a c h c a n b e a p p l i e d i n t h e d e s i g n o f new t w o e l e c t r o d e g a s - s e l e c t i v e sensors w i t h enhanced gas s e n s i t i v i t y . Such a d i f f e r e n t i a l gas s e n s i n g system i s composed o f two w o r k i n g g a s s e n s o r s , e a c h w i t h a d i f f e r e n t i n n e r i o n s e l e c t i v e p o l y m e r membrane e l e c t r o d e a s t h e t r a n s d u c e r . F o r e x a m p l e , a d i f f e r e n t i a l ammonia s e n s i n g arrangement i n v o l v e s t h e u s e o f two ammonia s e n s i n g probes whose


40



Recipient Buffer

toISE

SO^HSO-ZSO-Gas-permeable Membrane HS03/S03-- ^S0 ?

2

Plexiglass Dialysis Chamber Acidified Sample Solution

to Waste

Figure 7. F l o w - t h r o u g h g a s s e n s i n g a r r a n g e m e n t u s e d t o e v a l u a t e H g ( D D C ) 2 - P V C membrane e l e c t r o d e s f o r s e l e c t i v e d e t e c t i o n o f s u l f i t e s a s SO2.

L o g [sulfite],M Figure 8 . T y p i c a l response o f S O 2 g a ssensing system t o w a r d s u l f i t e s t a n d a r d s p r e p a r e d i n a b a c k g r o u n d o f 0.1 M sodium a c e t a t e . S a m p l e s a c i d i f i e d w i t h 0.1 M H3PO4; r e c i p i e n t stream; 0.1 M M E S b u f f e r , p H 6.0.



2. MEYERHOFF ET AL.


41

i n t e r n a l f i l l i n g solutions are connected v i a a salt bridge ( s e e F i g u r e 9 ) . One h a l f c e l l r e s p o n d s t o a m m o n i a g a s b y d e t e c t i n g a n i n c r e a s e i n pH i n a t h i n f i l m o f e l e c t r o l y t e ( i . e . , NH4CI) s a n d w i c h e d b e t w e e n a p o l y m e r i c p H s e n s i t i v e membrane ( p r e p a r e d w i t h t r i d o d e c y l a m i n e a s membrane a c t i v e species ( 9 ) ) a n d an o u t e r g a s p e r m e a b l e membrane. The s e c o n d h a l f c e l l d e t e c t s ammonia gas i n t h e s a m p l e b y r e s p o n d i n g t o c h a n g e s i n ammonium i o n a c t i v i t i e s i n a t h i n layer of buffer sandwiched between a nonactin-based ammonium i o n - s e l e c t i v e p o l y m e r i c membrane a n d a n o t h e r o u t e r g a s p e r m e a b l e f i l m (6) . Accordingly, the overall measured p o t e n t i a l f o r t h i s two w o r k i n g e l e c t r o d e c e l l i s t h e d i f f e r e n c e i n p o t e n t i a l b e t w e e n t h e ammonium i o n s e l e c t i v e e l e c t r o d e a n d t h e p o l y m e r i c pH electrode: Ε

= Ε cell

- Ε NH+

(2) pH

V

'

or E

ceii

= Κ + 0.059 l o g a

w h e r e Ε ., i s i n v o l t s ,

a

c e J.X

M U +

N H +

- 0.059 l o g a +

(3)

fl

and a + a r e t h e a c t i v i t i e s

NH.

of

M

4

ammonium i o n s a n d p r o t o n s i n the thin films of internal s o l u t i o n s h e l d between t h e o u t e r g a s p e r m e a b l e membranes and t h e r e s p e c t i v e i o n - s e l e c t i v e membranes, a n d Κ i s t h e sum o f a l l c o n s t a n t p o t e n t i a l s i n t h e c e l l ( e . g . , j u n c t i o n p o t e n t i a l s a t s a l t bridge, inner Ag/AgCl p o t e n t i a l s o f e a c h membrane e l e c t r o d e , e t c . ) . D i f f u s i o n o f gaseous ammonia i n t o t h e t h i n f i l m s r e s u l t s i n t h e e q u i l i b r i u m h y d r o l y s i s o f t h e ammonia: NH + H O NH++OH" (4) 3 2 4 w i t h an e q u i l i b r i u m constant

3

4

of

3

T h e r e f o r e , i f t h e pH o f t h e f i l m i n c o n t a c t w i t h t h e ammonium e l e c t r o d e i s b u f f e r e d , t h e a i n the film i s N H +

4

directly gas,

proportional to the partial

4

3

pressure

o f ammonia

3

On t h e o t h e r h a n d , f o r t h e p H e l e c t r o d e h a l f c e l l , since a ~ = K /a f then the a c t i v i t y of protons i n the thin f i l m i s given by, +

0 H

w

H


42

C H E M I C A L SENSORS AND

a + = H

However,

a

κ,'ΝΗ

NH ^ 1 4

i n t h e pH

half


(7) 3

cell,

a„„

i s kept

+

high

and

NH-


4

relatively constant by u s i n g N H 4 C I as t h e t h i n film electrolyte. Thus, s u b s t i t u t i n g e q u a t i o n s (6) a n d (7) into equation (3) a n d c o m b i n i n g a l l the constant terms together y i e l d s the following expression f o r the o v e r a l l differential cell potential, Ε (8) = Κ + 0.118 l o g P. NH cell 1

3

As c a n be s e e n , s u c h a c e l l pressure of ammonia w i t h mV/decade.

should respond a theoretical

to the slope

partial of 118

F i g u r e 10 i l l u s t r a t e s t h e r e s u l t s o b t a i n e d f o r s u c h a d i f f e r e n t i a l gas sensor cell arrangement. Varying concentrations of ammonia were generated by adding ammonium c h l o r i d e standards t o a 0.01 M NaOH sample solution. The s l o p e o f t h e d i f f e r e n t i a l s e n s o r i s 93.6 m V / d e c a d e i n t h e r a n g e o f 10"" -10~ M NH^ This less than 6

2

β

t h e o r e t i c a l v a l u e i s e x p e c t e d b a s e d on t h e d i f f i c u l t i e s i n e f f e c t i v e l y i s o l a t i n g a t h i n f i l m of s o l u t i o n at the t i p of each sensing h a l f c e l l from the bulk s o l u t i o n required for e l e c t r o l y t i c contact (30), and t h e f a c t t h a t t h e pH and ammonium p o l y m e r membranes t y p i c a l l y e x h i b i t subN e r n s t i a n b e h a v i o r ( e . g . , s l o p e s o f 5 2 - 5 6 mV d e c a d e ) . In addition, earlier theoretical predictions on t h e ammonia response o f t h e h a l f c e l l d e t e c t i n g ammonium i o n s a l s o p o i n t t o l o w e r t h a n t h e o r e t i c a l s l o p e s o w i n g t o s m a l l pH changes i n the b u f f e r e d f i l m (31). N a t u r a l l y t h e s e l e c t i v i t y o f t h e new d i f f e r e n t i a l g a s sensor design will be d e p e n d e n t on the combined gas selectivities of each gas sensing half-cell. Thus, enhanced s e n s i t i v i t y comes a t the expense of poorer s e l e c t i v i t y o w i n g t o t h e r e s p o n s e o f t h e pH s e n s i n g h a l f c e l l t o v o l a t i l e amines. However, f o r c e r t a i n t y p e s of samples (e.g. p h y s i o l o g i c a l f l u i d s ) t h i s should not pose a problem. Conclusions As d e s c r i b e d above, s i g n i f i c a n t progress in the d e s i g n o f a n i o n and gas s e l e c t i v e membrane e l e c t r o d e s has b e e n made. W h i l e f u r t h e r work i s needed t o understand fully the response mechanisms and to improve the performance of the new thiocyanate, salicylate, and sulfite selective membrane e l e c t r o d e s , e a c h o f these s e n s o r s appears t o o f f e r adequate s e l e c t i v i t y f o r use i n real sample measurements. In a d d i t i o n , by carefully


2. MEYERHOFF ET AL.

43



Ag/AgCI réf. electrode

ΐ Salt lonBridge selective Membrane NH;

NH =^NH£ 3

Gas —Permeable Membrane

NH =^OH~ 3

NH

ΝΗΛ

3

Figure 9. S c h e m a t i c o f d i f f e r e n t i a l a m m o n i a g a s s e n s o r fabricated w i t h two d i f f e r e n t polymer i o n - s e l e c t i v e membranes: ( a ) 0.1 M NH4CI; (b) 0.2 M p h o s p h a t e b u f f e r s p H 7 . 0 , c o n t a i n i n g 0.1 M N a C l ; ( c ) 0.1 M Tris-HCl b u f f e r , p H 7.8; (d) 0.05 M N H 4 C I .

too

-100

mV -200

300h

-400

_i

-8

I

-7

1

L

-6

-5

-4

-3

-2

log [ N H ] , M 3

F i g u r e 10. T y p i c a l c a l i b r a t i o n c u r v e f o r d i f f e r e n t i a l a m m o n i a s e n s o r s h o w n i n F i g u r e 9, i l l u s t r a t i n g e n h a n c e d ammonia g a s s e n s i t i v i t y .



44


studying the anion-coordinating p r o p e r t i e s o f d i f f e r e n t m e t a l - l i g a n d c o m p l e x e s i n p o l y m e r membranes, i t i s l i k e l y t h a t s e n s o r s f o r o t h e r i m p o r t a n t a n i o n s c a n be d e v i s e d . F u r t h e r m o r e , a s d e m o n s t r a t e d w i t h t h e new s u l f i t e s e n s o r , the development of anion responsive membranes a l s o d i r e c t l y i m p a c t s a d v a n c e s which c a n be made i n t h e a r e a o f gas s e l e c t i v e s e n s i n g s y s t e m s . Indeed, r e c o n f i g u r i n g t h e new s u l f i t e p o l y m e r i c membrane e l e c t r o d e a s a d e t e c t o r i n a f l o w - t h r o u g h gas phase s n i f f i n g arrangement (8) s h o u l d enable t h e continuous and s e l e c t i v e d e t e c t i o n o f t r a c e l e v e l s o f ambient SO2. The s e n s i t i v i t y o f t h i s a n d o t h e r gas s e n s i n g e l e c t r o d e s y s t e m s may be e n h a n c e d b y t a k i n g advantage of the novel differential cell concept, i n t r o d u c e d h e r e f o r t h e d e t e c t i o n o f d i s s o l v e d ammonia. Acknowledgments. The a u t h o r s gratefully acknowledge f i n a n c i a l support from t h e N a t i o n a l I n s t i t u t e s o f H e a l t h (GM-28882) a n d M a l l i n c k r o d t S e n s o r S y s t e m s . The a u t h o r s a l s o w i s h t o t h a n k P r o f e s s o r John T. G r o v e s , Department o f Chemistry, P r i n c e t o n U n i v e r s i t y , f o r p r o v i d i n g s e v e r a l o f the m e t a l l o p o r p h y r i n s t r u c t u r e s used i n these s t u d i e s .

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