DEVELOPMENT OF ELECTROSPUN NANOFIBER MODIFIED DISPOSABLE ELECTRODE FOR BIOSENSOR APPLICATIONS

P o n g p o l E k a b u tr

A D is s e r ta t io n S u b m it te d in P a r t ia l F u l f i lm e n t o f th e R e q u ir e m e n ts f o r th e D e g re e o f D o c to r o f P h i lo s o p h y

T h e P e tr o le u m a n d P e tr o c h e m ic a l C o l le g e , C h u la lo n g k o r n U n iv e rs i ty in A c a d e m ic P a r tn e r s h ip w ith

T h e U n iv e r s i ty o f M ic h ig a n , T h e U n iv e r s i ty o f O k la h o m a , a n d C a s e W e s te rn R e s e r v e U n iv e rs i ty

2 0 1 4

T 'f t ร า r\ I\ An

Thesis Title; D e v e lo p m e n t o f E le c tro sp u n N a n o fib e r M o d ifie d D isp o sa b le E le c tro d e fo r B io se n so r A p p lic a tio n s

By: P on g p o l E k ab u trProgram: P o ly m e r S c ien ceThesis Advisors: P ro f. P itt S u p ap h o l

A c c e p te d b y T h e P e tro leu m an d P e tro c h e m ic a l C o lle g e , C h u la lo n g k o rn U n iv e rs ity , in p a rtia l fu lf ilm e n t o f th e re q u ire m e n ts fo r th e D eg ree o f D o c to r o f P h ilo so p h y .

C o lle g e D ean

Thesis Committee:

(A ss t. P ro f. P o m th o n g M alak u l) (P ro f. P itt S u p ap h o l )

....... .......................................................................................................................zfjLL....................

(P ro f. O ra w o n C h a ila p a h u l) (P ro f. A n u v a t S iriv a t)

...........^ .......................................... ..........ร(A ss t. P ro f. T h a n y a la k C h a isu w a n ) (D r. P im o lp u n N ia m la n g )

r i o O i i

Ill

ABSTRACT

5 3 9 2 0 0 1 0 6 3 : P o ly m e r S c ie n c e P ro g ra mP o n g p o l E k a b u tr : D e v e lo p m e n t o f E le c t ro s p u n N a n o f ib e r M o d if ie d D is p o s a b le E le c t ro d e fo r B io s e n s o r A p p l ic a t io n s .T h e s i s A d v is o r s : P ro f . P i t t S u p a p o l 145 p p .

K e y w o rd s : E le c t r o s p in in g / N a n o f ib e r / G r a p h e n e / C a r b o n n a n o tu b e / G lu c o s e /D o p a m in e / C a r b o n s c r e e n - p r in te d e le c t r o d e / V a p o r - p h a s e / P o ly m e r i z a t io n / C a r b o n iz a t io n

A n e w ty p e o f b io s e n s o r w a s f a b r ic a te d u s in g a c a r b o n iz e d h y b r id g o ld ( A u ) /g r a p h e n e (G ) n a n o w ir e a n d v a p o r - p h a s e p o ly m e r iz a t io n o f c o n d u c t iv e p o ly m e r c o n s t r u c te d o n a d i s p o s a b le s c r e e n - p r in te d c a r b o n e le c t r o d e ( S P C E ) . E le c t r o s p in n in g , c a r b o n iz a t io n a n d c o n d u c t iv e p o ly m e r ic c o a t in g p r o c e s s e s w e r e c o m b in e d to a c h ie v e th e s e le c t iv e a n d s e n s i t iv e d e te r m in a t io n o f b io m o le c u le s s u c h a s g lu c o s e (G U ) a n d d o p a m in e (D A ) . S c a n n in g e le c t r o n m ic r o s c o p y ( S E M ), t r a n s m is s io n e le c t r o n m ic r o s c o p y (T E M ) a n d X - ra y d i f f r a c t io n ( X R D ) w e r e u s e d to c h a r a c te r iz e th e s u r fa c e m o r p h o lo g y a n d p h y s ic a l p r o p e r t ie s o f th e e le c t r o s p u n p r o d u c ts . T h e b a s ic c h a r a c te r iz a t io n o f e le c t ro c h e m ic a l b e h a v io r o f th e v a r io u s m o d if ie d e le c t r o d e s in [ F e ( C N ) 6 ]3"/4‘ w a s s tu d ie d b y c y c l ic v o l ta m m e tr y ( C V ) a n d E le c t r o c h e m ic a l im p e d a n c e s p e c t ro s c o p y (E IS ) . T h e r e s u l t s s h o w th a t m o d if ie d e le c t r o d e s e x h ib i te d d r a s t i c a l ly h ig h c u r r e n t r e s p o n s e c o m p a re d to u n m o d i f ie d e le c t r o d e . M o re o v e r , th e v a r io u s m e th o d s o f e le c t r o c h e m ic a l s tu d ie s ; a m p e r o m e tr y ( A P M ) , s q u a re - w a v e v o l ta m m e tr y ( S W V ) a n d d i f f e r e n t ia l p u ls e v o l ta m m e tr y ( D P V ) w e r e u s e d to s y s te m a t ic a l ly m e a s u r e a n d o p t im iz e th e o x id a t io n c u r r e n t o f b io lo g ic a l a n a ly s e s . F in a l ly , th e l in e a r c u r r e n t r e s p o n s e to b io lo g ic a l c o n c e n t r a t io n s , s e n s i t iv i ty , l im it o f d e te c t io n (L O D ) a n d in te r f e r in g s tu d ie s . T h e m o d if ie d e le c t r o d e c o u ld b e a p r o m is in g c a n d id a te f o r u s e a s a h ig h - p o te n t ia l e le c t r o d e , r e p r e s e n t in g a n e w a p p r o a c h f o r th e s e le c t iv e a n d s e n s i t iv e d e te r m in a t io n o f b io lo g ic a l p r o d u c ts w i th lo n g - te r m s e n s o r s ta b i l i ty .

IV

บทคัดย ่อ

พ งศ ์พ ล เอ ก บ ุต ร : ก าร พ ัฒ น าว ัส ด ุต ร ว จ จ ับ ส าร ช ีว ภ าพ ด ้ว ย เส ้น ใย ร ะ ด ับ น าโ น ท ี่ผ ่าน

ก ารข ึ้น ร ูป ด ้ว ย ก ร ะ บ ว น ก าร ป ่น เส ้น ใย ไ ฟ ฟ ้าส ถ ิต ย ์บ น ฃ ัว ไ ฟ ฟ ้าแ บ บ ใช ้แ ล ้ว พ ิง ( D e v e lo p m e n t o f E le c t r o s p u n N a n o f ib e r M o d i f ie d D is p o s a b le E le c t r o d e fo r B io s e n s o r A p p l ic a t io n s ) อ าจารย ์ท ี่ป ร ึก ษ า : ศ .ด ร .พ ิช ญ ์ ศ ุภผล 131 หน ้า

การพัฒนาวัสดุตรวจจับสารชีวภาพด้วยเทคนิค คาร์บอนไนไซชัน (carbonization) และ การเคลือบผิวเส ้นใยด้วยพอณิมอร์นำไฟฟ้าในสถานะไอ (vapor-phase polymerization) บน ขั้วไฟฟ้าคาร์บอนแบบใช้แล้วทิ้ง โดยกระบวนการดังกล่าวสามารถนำมาใช ้ในการปรับปรุงผ ิว ขั้วไฟฟ้า ส ่งผลให้ข ั้วไฟฟ้ามีความว่องไวต่อปฏิกิร ิยาทางไฟฟ้าเคมีของสารตรวจวัด เช่น นํ้าตาล กลูโคสและโดพามีน ได้อย่างมีประสิทธิภาพ เส้นใยระดับนาโนที่พัฒนาขึ้นนี้จะถูกศึกษาสมปติทาง กายภาพดวยเทคนิค Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) และ X-ray diffraction (XRD) ตามลำดับ หลังจากนั้นเส้นใยนาโนจะถูกนำไปปรับปรุง พืนผิวของขั้วไฟฟ้าทำงาน โดยขั้วไฟฟ้าที่ผ ่านการปรับปรุงผิวจะถูกศึกษาพฤติกรรมทางไฟฟ้าเคมี โดยใช้เทคนิคไซคลิกโวแทมเมททรี (cyclic voltammetry) และอิมพิแดนซ์อิเล ็กโทรสโคปี (electrochemical impedance spectroscopy) จากผลการทดลองพบว่าขัวไฟฟ้าทีผ่านการปรับปรุงผิว ด้วยเส้นใยนาโนมีการตอบสนองต่อสัญญาณไฟฟ้าที่ส ูงกว่าขั้วไฟฟ้าที่ไม่ได้ผ ่านการปรับปรุงผิว อย่างชัดเจน นอกจากนีเทคนิคกระบวนการวัดทางไฟฟ้าเคมีอื่นๆ เช่น เทคนิคแอมเพอโรเมททริ (amperometry) เทคนิคสแควเวฟ โวแทมเมททร (square-wave voltametry) และเทคนิคดิฟเฟอเรน เชียลพัลส์โวลทาเมตรี (differential pulse voltametry) ได้ถูกนำมาใช้ในการศึกษาปฏิกิริยาออกซิ- เดชั่นของสารตรวจวัดชีวภาพอย่างเหมาะสม โดยในงานวิจัยจะทำการศึกษา ค่าช่วงความเข้มข้น ของการตรวจวัดที่มีความสัมพันธ์เชิงเส้นตรง (linear current response) ความว่องไวของการ ตรวจวัค (sensitivity) ค่าขีดจำกัดของการตรวจวัด (limit of detection) และสภาพแวดส้อมที่รบกวน ต่อสัญญาณการตรวจวัด (interfering studies) ซึ่งจากผลการทดลองทั้งหมดจะเห็นได้ว่า ขั้วไฟฟ้าที่ ผ ่านการปรับปรุงด้วยเส ้นใยระดับนาโนด้วยวิธ ีต ่างๆ มีศ ักยภาพในการนำมาพัฒนาขั้วไฟฟ้าท ี่ม ี ความว่องไวและมีความเสถียรภาพสูง

V

ACKNOWLEDGEMENTS

T h is w o r k w a s s u p p o r te d in p a r t b y (1 ) T h e C h u la lo n g k o m U n iv e r s i ty D u ts a d i P h ip h a t S c h o la r s h ip , a n d (2 ) P ro f . O r a w o n C h a i la p a k u l . E le c t ro c h e m is t r y a n d O p t ic a l S p e c t r o s c o p y r e s e a r c h u n i t , F a c u l ty o f S c ie n c e , C h u la lo n g k o m U n iv e r s i ty , T h a i la n d . W e a re th a n k fu l to P ro f . S c h w a n k ’ร r e s e a rc h g r o u p . U n iv e r s i ty o f M ic h ig a n , U S A fo r th e u s e o f th e i r m u f f le o v e n .

F in a l ly , I w o u ld l ik e to th a n k to m y a d v is e r ; P ro f . P i t t S u p a p h o l a n d P ro f . O r a w o n C h a i la p a k u l fo r e v e r y th in g in c lu d in g r e s e a rc h fu n d s a n d u s e fu l s u g g e s t io n s w h ic h c a n d r a s t i c a l ly s u p p o r t m y r e s e a rc h e s .

T h is th e s is w o rk is f u n d e d b y T h e P e tr o le u m a n d P e t r o c h e m ic a l C o l le g e ; a n d th e N a t io n a l C e n te r o f E x c e l le n c e f o r P e tr o le u m , P e tr o c h e m ic a ls , a n d A d v a n c e d M a te r ia ls , T h a i la n d .

TABLE OF CONTENTS

T it le P a g e iA b s t r a c t ( in E n g l is h ) iiiA b s t r a c t ( in T h a i ) ivA c k n o w le d g e m e n ts V

T a b le o f C o n te n ts v iL is t o f T a b le s ixL is t o f F ig u r e s x i

CHAPTERI INTRODUCTION 1

II THEORETICAL BACKGROUND AND LITERATURESURVEY 52.1 T h e o re t ic a l B a c k g r o u n d 52 .2 L i te r a tu re S u rv e y 3 2

III MODIFICATION OF DISPOSABLE SCREEN-PRINTEDCARBON ELECTRODE SURFACES WITH CONDUCTIVE ELECTROSPUN NANOFIBER FOR BIOSENSOR APPLICATIONS 3 73.1 A b s t r a c t 3 73 .2 I n tr o d u c t io n 383 .3 E x p e r im e n ta l 3 93 .4 R e s u l ts a n d D is c u s s io n 4 33 .5 C o n c lu s io n s 51

PAGE

IV DEVELOPMENT OF DISPOSABLE ELECTRODE MODIFIEDWITH CARBONIZED, ELECTROSPUN, POLYACRYLONITRILE-LOADED GRAPHENE NANOPARTICLES FOR THE DETECTION OF DOPAMINE IN HUMAN SERUM 664.1 A b s t r a c t 6 6

4 .2 I n tr o d u c t io n 6 74 .3 E x p e r im e n ta l 6 8

4 .4 R e s u l ts a n d D is c u s s io n 7 24 .5 C o n c lu s io n s 7 7

V NOVEL CARBONIZED, GOLD/GRAPHENE HYBRIDNANOWIRE-MODIFIED DISPOSABLE ELECTRODE FOR THE ULTRASENSITIVE ANS SELECTIVE DETECTION OF DOPAMINE IN BIOLOGICAL SAMPLES 9 05.1 A b s t r a c t 9 05 .2 I n tr o d u c t io n 915 .3 E x p e r im e n ta l 935 .4 R e s u l ts a n d D is c u s s io n 9 65 .5 C o n c lu s io n s 102

VI DEVELOPMENT OF A NOVEL ANTI-TUBERCULOSIS MELT-BLOWN POLYPROPYLENE FILTER COATED WITH MANGOSTEEN EXTRACTS FOR MEDICAL FACE MASK APPLICATIONS 1136.1 A b s t r a c t 1136 .2 I n t r o d u c t io n 1136 .3 E x p e r im e n ta l 1156 .4 R e s u l ts a n d D is c u s s io n 11 9

vii

CHAPTER PAGE

V l l l

CHAPTER PAGE6 .5 C o n c lu s io n s 120

VII CONCLUSIONS AND RECOMMENDATIONS 1267.1 C o n c lu s io n s 12 67 .2 R e c o m m e n d a t io n s 12 7

REFERENCES 128

CURRICULUM VITAE 143

IX

TABLE PAGE

CHAPTER II2.1 P ro p e r ty c h a n g e s ty p ic a l ly o b s e r v e d u p o n e le c t r ic a l

s t im u la t io n to s w i tc h C P s b e tw e e n o x id iz e d a n d r e d u c e ds ta te s 7

2 .2 T h e a p p l ic a t io n s o f b io s e n s o r 2 7

CHAPTER III3.1 C o m p a r i s o n o f m o d if ie d e le c t ro d e s w i th o th e r s G O X b a s e d

a m p e r o m e tr ic g lu c o s e b io s e n s o r s r e p o r te d in l i te r a tu r e 65

CHAPTER IV4 .1 E le c t r o c h e m ic a l d a ta fo r e a c h m o d if i e d e le c t r o d e 8 8

4 .2 C o m p a r i s o n o f th e p r o p o s e d e le c t ro d e to o th e r m o d if ie de le c t r o d e s fo r D A d e te c t io n g g

CHAPTER V5.1 E le c t r o c h e m ic a l d a ta fo r e a c h m o d if ie d e le c t r o d e 1115 .2 C o m p a r i s o n o f th e p r o p o s e d e le c t ro d e to o th e r m o d if ie d

e le c t r o d e s fo r D A d e te c t io n U 2

CHAPTER VI6.1 T he c h a rac te ris tic p ro p ertie s o f p o ly p ro p y len e m elt-b lo w n filte rs J 2 46 .2 T he filtra tio n pe rfo rm an ces o f co m p le te face m ask

LIST OF TABLES

X

LIST OF FIGURES

FIGURE PAGECHAPTER II

2.1 E le c t r o s p in in g p r o c e s s 5

2 .2 T y p ic a l c o n d u c t in g p o ly m e r s tru c tu re s . 8

2 .3 S y n th e s i s C P s v ia c h e m ic a l p o ly m e r iz a t io n 9

2 .4 C h e m ic a l p o ly m e r iz a t io n v ia r a d ic a l m e c h a n is m 10

2 .5 S c h e m a tic d r a w in g o f p o ly m e r iz a t io n c h a m b e r o f v a p o r -p h a s ep o ly m e r iz a t io n 1 1

2 .6 D e r iv a t iv e s o f s u l fo n ic a c id u s e d a s o x id iz in g a g e n ts f o r v a p o r -p h a s e p o ly m e r iz a t io n 1 2

2 .7 G e n e ra l s t r u c tu r e o f a n a m in o a c id ; th e s u b s t i tu e n t g r o u p (R )v a r ie s f ro m o n e a m in o a c id to a n o th e r . 13

2 .8 P e p t id e b o n d fo rm a t io n f ro m tw o a m in o a c id s . 13

2 .9 D o u b le b o n d c h a r a c te r o f th e C -N b o n d in p e p tid e . 14

2 .1 0 T h e u t i l i z a t io n o f e n z y m e in c a ta ly t ic c h e m ic a l re a c t io n s . 15

2 .11 D ia g r a m s to s h o w th e in d u c e d f i t h y p o th e s i s o f e n z y m eR e a c t io n 16

2 .1 2 C o m p e t i t iv e in h ib i to r s b in d r e v e r s ib ly to th e e n z y m e ,p r e v e n t in g th e b in d in g o f s u b s tra te . 17

2 .1 3 “ Z e ro o r d e r ” r e a c t io n r a te in v o lv e d w ith in d e p e n d e n t

o f s u b s tr a te c o n c e n t ra t io n . 18

XI

2.14 Effect of substrate concentration on the velocity of an enzyme. 182.15 Effect of temperature on reaction rate. 192.16 Depemdence of rate on substrate concentration. 192.17 Michaelis-Menten plot relating the reaction rate V to the

substrate concentration. 2 22.18 Hanes-Woolf plot relating the substrate/v to 1/substrate 22

concentration.2.19 Schematic representation of a biosensor and factors defining

the sensor signal. 232.20 Operating principle of the amperometric detection with

mediator. 262.21 Operating principle of the amperometric detection with

mediator. 262.22 Cyclic voltammogram of ferrocene.The inset shows the

definition of scan rate. 282.23 Square wave potential sweep. 292.24 Simple electrode reactions. 302.25 Graphene is an atomic-scale honeycomb lattice made of carbon

atoms. 31

CHAPTER III3.1 Schematics of electrode fabrications. 523.2 Effect of % MWCNT loading on fiber diameter. 523.3 The samples of SPCE coated with PAN-MWCNT nanofiber at

various times. 5 33.4 Anodic current response of modified electrodes. 533.5 (a) Effect of concentration of FeTos oxidant on fiber diameter

and (b) Effect of concentration of FeTos oxidant on anodiccurrent response of 5 mM Fe 2+/3+ in 0.1 M PBS of pH 7.4. 54

FIGURE PAGE

3.6 SEM images of modified SPCE coated with PPy layer byvapor-phase polymerization at various times. 55

3.7 SEM images of the surface morphologies of (a) SPCE, (b) PAN-MWCNT/SPCE, and (c) PPy/PAN-MWCNT/SPCEelectrodes. 55

3.8 FT-IT spectra of PAN-MWCNT/SPCE (bottom line) andPPy/PAN-MWCNT/SPCE (top line). 55

3.9 (a) Cyclic voltammograms of different modified electrodes measured 5 mM Fe2+/3+ in 0.1 M PBS of pH 7.4 at a scan rate of 50 mV/ร (b) Cyclic voltammograms of BARE-CNT-PPy electrode at different scan rate, (inset) plot linear relationcurrent vs. scan ratel/2. 56

3.10 (a) Cyclic voltammograms of electrodes of 10 mM H2O2, 0.1 M PBS of pH 7.4 at 50 mV/ร (b) Amperometric response of electrodes of 10 mM H2O2 in 0.1 M PBS of pH 7.4 potential fixed at +1.4 V for SPCE and PAN-MWCNT/SPCEand +1.2 Vfor PPy/PAN-MWCNT/SPCE, respectively. 57

3.11 Amperometric responses of PPy/PAN - M WCNT/S PC E electrode to sequential increase of glucose in 0.1 M PBS of pH7.4. 58

3.12 Cyclic voltammogram and anodic current plots of PPy/PAN- MWCNT/SPCE with various pH of supporting electrolyte for10 mM H2O2 detection. 58

3.13 Cyclic voltammogram and anodic current plots of PPy/PAN- MWCNT/SPCE with various KC1 concentrations for 10 mMH2O2 detection. 59

3.14 Cyclic voltammogram and anodic current of PPy/PAN- MWCNT/SPCE with various supporting electrolyte concentrations for 10 mM H2O2 detection.

xii

FIGURE PAGE

59

X ll l

3.15 Amperometric peak current of 1 mM glucose in various GOX 55concentrations.

3.16 Amperometric measurements obtained for standard glucose additions using an applied potential of + 1.25 V with BARE- CNT-PPy electrode. Inset is a calibration curve of BARE-CNT-PPy electrode. 60

3.17 Cyclic voltammogram of SPCE and PPy/PAN-MWCNT/SPCEin 50 mM mediator system. 61

3.18 Cyclic voltammogram of PPy/PAN-M WCNT/SPCEincorporation with 0.1 g/ml of GOX and various mediator concentrations. 61

3.19 Amperometric responses of PPy/PAN-MWCNT/SPCE electrode incorporate with various mediator concentrations todetect 1 mM of glucose 62

3.20 Amperometric measurement obtained for standard glucoseadditions 63

3.21 Hanes plot for comparison of GOX /PPy/PAN-MWCNT/SPCE(top) and GOX+Mediator/PPy/PAN- MWCNT/SPCE(bottom). 64

CHAPTER IV4.1 Schematic of SPCE surface modifications. 784.2 (a) SEM image of PAN5G before carbonization and

(b) SEM image of CPAN5G after carbonization. 794.3 XRD patterns of G, PAN, PAN5G, and CPAN5G nanofibers. 804.4 c v images of unmodified/modified electrodes with a scan rate

of 50 mV ร' 1 in 0.1 M PBS at a pH of 7.4, (a) in 1 mM [Fe(CN)6]3'/4‘ and (b) in 40 pM DA.

FIGURE PAGE

81

XIV

4.5 c v images of the CPAN5G-4x electrode measured using 1 mM [Fe(CN)6]3'/4‘ in 0.1 M PBS at a pH of 7.4 at different scan rates. Inset is the plot of the linear relationship between the current and scan rate172 for the bare and CPAN5G-4x electrodes.

4.6 EIS of BARE, CPAN, CPAN5G, and CPAN5G-4x in 1 mM [Fe(CN)6]3"/4" in the presence of 0.1 M PBS at a pH of 7.4.

4.7 SWT profiles of the unmodified and modified electrodes in 0.1 M PBS/5 mM SDS at a pH of 2. s w v detection conditions: pulse amplitude=0.06 V, square wave frequency=18 Hz, and step height=0.005 V.

4.8 SWT profiles of the unmodified and modified electrodes.4.9 (a) Representative sw v profiles of the CPAN5G-4x electrode

in 0.1 M PBS/5 mM SDS at a pH of 2; The inset of (a) is the calibration curve for DA detection over a concentration range of 0.5-100 pM in the presence of AA and UA at 80 and 400 pM, respectively; (b) Representative SWT profiles of the CPAN5G-4x electrode in human serum after TCA protein precipitation; The inset of (b) is the calibration for different spiked DA concentrations over the range of 0.5-100 pM.

4.10 Determination of Ipadpc, Epa and Epc of CPAN5G-4x electrodes with a scan rate of 50 mV ร' 1 in 0.1 M PBS at a pH of 7.4 in 1 mM [Fe(CN)6]3'/4'.

4.11 SWV profiles of the CPAN5G-4X electrode the presence of 40 pM DA, 80 pM AA, and 400 pM in 0.1 M PBS/5 mM SDS at various pH conditions, sw v detection conditions: pulse amplitude=0.06 V, square wave frequency=18 Hz, and step height=0.005V.

FIGURE

82

83

8485

PAGE

8 6

87

87

XV

FIGURE

CHAPTER V

PAGE

5.1 Schematic of SPCE surface modification. 1045.2 (a) SEM image of PAN-Au/G before carbonization, (b) SEM

image of CPAN-Au/G after carbonization, (c) TEM image of CPAN-Au/G, and (d) EDX microanalysis spectrum of CPAN-Au/G. 105

5.3 XRD patterns of graphene, PAN, PAN-G, PAN-Au/G, andCPAN-Au/G nanofibers. 106

5.4 Electrochemical studies of unmodified/modified electrode. 1075.5 DPV profiles of the CPAN-Au/G electrode in the presence of

40 pM DA in 0.1 M PBS at various pH levels. The insets are (a) current density of CPAN-Au/G at various pH levels and (b)the plots of oxidation potential at various pH levels. 108

5.6 DPV profiles of electrodes in the presence of 40 pM DA, 80 pM AA, and 400 pM UA in 0.1 M PBS at pH 7.4; (a) bareSPCE and (b) CPAN-Au/G electrode. 109

5.7 Representative DPV profiles of the CPAN-Au/G electrode in 0.1 M PBS at pH 7.4. The inset of the calibration curve for DA detection over a concentrationrange of 0-60 pM.

CHAPTER VI

1 1 0

6 .1 SEM images of polypropylene melt-blown filter (a) beforecoating and (b) after coating with 5% (w/v) of MG solution. 1 2 2

6 .2 Percentage of E.coli reduction as a function of time interval. 1236.3 Percentage of ร. aureus reduction as a function of time interval. 1236.4 Percentage of MDR-M. tuberculosis reduction as function of

time intervals. 124