Potential Modulation on Total Internal Reflection Ellipsometry

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Potential Modulation on Total Internal Reflection Ellipsometry Wei Liu, Yu Niu, Ana S. Viana, Jorge Palma Correia, and Gang Jin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04587 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 24, 2016

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Analytical Chemistry

Potential Modulation on Total Internal Reflection Ellipsometry Wei Liu,

†, ‡, ¶, ⊥

Yu Niu,

†, ‡, ⊥

§

A. S. Viana, Jorge P. Correia,

§

and Gang Jin*, ‖, ‡

†NML, Institute of Mechanics, Chinese Academy of Sciences, # 15, Bei-si-huan West Road, Beijing, 100190, China ‡Beijing Key Laboratory of Engineered Construction and Mechanobiology, Institute of Mechanics, Chinese Academy of Sciences, 15 Bei-si-huan West Road, Beijing 100190, China ¶University of Chinese Academy of Sciences, # 19, Yu-quan Road, Beijing, 100049, China §Centro de Química e Bioquímica, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal ‖NML, Institute of Mechanics, Chinese Academy of Sciences, # 15, Bei-si-huan West Road, Beijing, 100190, China ⊥Contributed equally to this work Email: [email protected] Fax: +86-10-82544138

ACS Paragon Plus Environment


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Electrochemical-Total internal reflection ellipsometry (EC-TIRE) has been proposed as a technique to observe the redox reactions on the electrode surface due to its high phase sensitivity to the electrolyte / electrode interface. In this paper, we mainly focus on the influence of the potential modulation on the TIRE response. The analysis suggests that both dielectric constant variation of gold and the electric double layer transformation would modulate the reflection polarization of the surface. For a non-faradaic process, the signal of TIRE would be proportional to the potential modulation. To testify the analysis, linear sweep voltammetry and open circuit measurement have been performed. The results strongly support the system analysis.

Introduction Total internal reflection ellipsometry (TIRE) biosensor has been proposed as a real-time, label-free tool to study the biomolecule interactions 1,2, because of its high sensitivity to various physical, chemical and biochemical processes at the interface. The high interface sensitivity of TIRE arises from the evanescent field under the total internal reflection condition. In the evanescent field, a plane wave, so-called evanescent wave, propagates along the interface, while its amplitude decays exponentially in the direction normal to the interface. The decay distance from the interface to the medium with lower refractive index is less than the wavelength 3. To gain better sensitivity, a thin metal film is introduced to the TIRE sensor chip, and thus, the amplitude of the evanescent wave will be enhanced by the surface plasmon resonance (SPR) 4. However, compared with SPR technique, TIRE achieves higher sensitivity, because TIRE not only enjoys the information of which is equivalent to the SPR response, but that of the ellipsometric phase Δ which is shown higher susceptibility to the surface refractive index and layer thickness than 5.

Since TIRE can obtain the precise refractive index and thickness of the formed layer, it is employed as a powerful tool to study in situ protein adsorption on the sensing surface 1,6-8, low molecular weight environmental toxin registration 9-11, genomic DNA adsorption and subsequent interactions 12,13, disease diagnosis 14,15 and weak affinity among biomolecules 16,17.

On the other

hand, for many biochemical applications, aqueous electrolyte is used as buffer. Because of the inner potential difference between the two phases, an electric double layer (EDL) would be formed at the interface. Furthermore, the interfacial potential difference will change when the charge equilibrium of the interface is broken by the adsorption of such biomolecules as proteins, DNA and DNA fragments 18,19. On the other hand, an external electric field will also influence the biomolecule interactions 20-22. Aiming to modulate the applied potential during a TIRIE biosensor experiments, the homemade TIRE equipment has been combined with an external electrochemical workstation, and the Electrochemical-TIRE (EC-TIRE) has been proposed 23-25 As is discussed in previous works 26-30, the interfacial potential not only exerts influences on complex refractive index of the liquid phase, but also on that of the metal film. Thus, the optical response of TIRE biosensor will be modulated when the potential varies. It is not addressed whether this modulation is beneficial to the biomolecule detection of TIRE biosensor in the previous works. In fact, this modulation might cause the false signal for the biomolecule interaction detection in practice. Further, the working conditions for the EC-TIRE biosensor should be elucidated. In this paper, we have studied the potential modulation influence on the TIRE response. Two most used electrochemical methods are analyzed: linear sweep voltammetry and open circuit potential techniques.


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Methodology and Experimental The Sensitivity of TIRE As EC-TIRE uses the conventional polarizer-compensator-sample-analyzer (PCSA) configuration,

the fractional change / of the optical response is given by (see Supporting Information for deduction)

=

+ + Δ

(1)

where is the reflectance of the surface for p-polarized light, given by

=

and are the coefficients for the ellipsometric parameter variations, and Δ. The explicit expressions are given by $ % 2tan 1 + cos 2 − sin 2 sin2# + Δ 4 − $ sin 2% sin 2 1 − cos 2 cos 2 + sin 2 sin2# + Δ $ sin 2 − sin 2 cos2# + Δ

= $ sin 2% 1 − cos 2 cos 2 + sin 2 sin2# + Δ

=

$ are unperturbed values of the ellipsometric parameters and Δ and # and where and Δ are the polarizer and analyzer azimuths respectively. Eq. ( 1 ) implies that by introducing the gold film on TIRE system, the optical signal can be categorized in two parts: the response from SPR and that from the reflection polarization modulation of the surface. By proper polarization configuration, the later will give TIRE a better sensitivity as is reported in literature 9,31. In fact, since the sensitivity of TIRE is at least 10 times larger than that of SPR, we may conclude that the high sensitivity of TIRE mainly originates from the polarization modulation rather than SPR phenomenon. On the other hand, Kyle J. Foley, Xiaonan Shan, and N. J. Tao have discussed the influence of the potential variation on SPR response 32. Thus, we mainly focus on its influence on the ellipsometric

parameter ' (' = /( ).

The Potential Modulation Influence on Gold Dielectric Constant. For a non-faradaic process, a potential change would cause the EDL transformation in the vicinity

of the electrode. The relationship between the surface charge density change, ), and the potential

modulation on the electrode, *, is given by 33:

) = +*

(2)

where + is the differential capacitance density (capacitance per unit area) of the interface, which is determined by the electrolyte concentration, its nature and the interface potential. On the other hand, for a thin gold film of thickness , , a surface charge density change will exert

an influence on the dielectric constant of the film, - , by 27 - = −- − 1

.

/01 23

= - − 1 4

(3) .

where 5 is the electron charge, 6/ the electron density and 4 = − /0

1 23

, the electron density

change. For simplicity, we can use 4 to describe the dielectric constant change of the metal since it

is real, while - is usually complex.



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EDL Influence Besides the gold dielectric constant variation, EDL also influences the dielectric constant of the

electrolyte in the neighborhood of the electrode, - . The dielectric constant of the electrolyte can be

given by 34

C

A

A

- = 7D ∑ 9 :9 ;, = exp − B , B

(4)

where :9 ;, = is the instant local concentration of ion i, 9 the dielectric constant change per unit concentration for ion i. The decay of evanescent wave from the metal into the liquid phase is described by the exponential term in the integral and the decay length l, is around 100 nm. Although, in principle, l is also a function of ion i concentration, a first order of approximation makes l a constant. For a non-faradaic process, Gouy-Chapman theory predicts the concentration of ion i in the

neighborhood of the electrode, :9 ;, = , can be given by 35 :9 ;, = = :9D exp E−

FG / HI

J;, = K

(5)

where :9D is the bulk concentration of ion i, L9 the (signed) charge on ion i, M Boltzmann

constant, N the absolute temperature and J;, = the potential near the electrode. Especially, at the

electrode, where ; = 0, we have

:9 0, = = :9D exp P−

FG /

J0, = Q = :9D exp E−

HI

FG / HI

*K

(6)

Expanding :9 ;, = by Taylor expansin at ; = 0, taking it to eq. (4) and ignoring the high-order terms, we have - ≈ ∑ 9 :9 0, =

(7)

According to eqs. (6) and (7), the potential disturbance would modulate - by F/

G - = ∑ 9 :9 0, = = ∑ 9 P− HI :9 0, = Q *

(8)

High Polarization Sensitivity around SPR Angle According to Supporting Information, the influence of the gold dielectric constant modulation

- and the electrolyte dielectric constant variation - on the ellipsometric parameter ' can be given by S S

= T - + T -

(9)

On the other hand, we have ' = tan 5 UV

( 10 )

By taking logarithmic differential of eq.(10), we obtain S S

= WXY Z + [Δ

( 11 )

Thus,

S

= E sin 2K Re E K

S

Δ = Im E K

S

S


( 12 )

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In general, because of the damping, the relative dielectric constant of the metal is complex. Taking eqs. (3) and (9) into eqs. (12), we get 1 1 = P sin 2Q ReT - + T - = P sin 2Q _Re`T - − 1 a4 + ReT - b 2 2 Δ = ImT - + T - = Im`T - − 1 a4 + ImT -

( 13 )

By eqs. (2), (3), (8) and (13), we have ∝ *, Δ ∝ *

( 14 )

which means the ellipsometric parameter changes are proportional to the potential disturbance of the sensing surface. Since 32 has proven that SPR response is also proportional to the potential disturbance, according to eqs. (1) and (14), TIRE signal change would be proportional to the potential modulation for a non-faradaic process, i.e., ∝ *

( 15 )

T in eq. (9) suggests to what extent the minor variation of the metal dielectric constant

modulates on the polarization of the reflection: ReT - shows the sensitivity of to gold

dielectric constant and ImT - the sensitivity of Δ. Likewise, ReT - demonstrates the

sensitivity of to electrolyte dielectric constant, ImT - the sensitivity of Δ to electrolyte

dielectric constant. To compare the influence of the potential modulation on the gold film and the electrolyte, 4 in

eq. (3) and - need to be estimated. By definition and eq. (2), 4 can be given by 4 = −

.

( 16 )

/01 23

For most electrode/solution systems, the capacitance density is around 10 ~ 40 ef/+g

35

. The

potential modulation is less than 0.1 * for biosensor applications. The thickness of the gold film is

around 50 jg and the electron density of the metal is around 10k glm . 4 is estimated around 10lm , while the order of the electrolyte dielectric constant variation - is estimated around 10lm , according to 29.

Figure 1: When the dielectric constants of the gold and the electrolyte change, the ellipsometric parameters variations, (a) and (b) Δ, change with the angle of incidence JD . n = 632.8 jg,

-rsD = 2.969, -uv = −10.60 + 1.28[,-/B/wxyzB{x/ = 1.769.

On the other hand, ellipsometric parameters are also modulated by the angle of incidence, since

both T and T in eq. (9) are the functions of angle of incidence. Fig. 1 suggests when the dielectric

constants of the gold and the electrolyte changes, the ellipsometric parameters, and Δ, are the most sensitive around SPR angle which can be estimated by sin }y = ~

1

1

, around 57.8°. The

36

sensitivity of Δ, reaches the maximum at SPR angle. Contrarily, because of the resonance

absorption, = 0 at SPR angle, however, in the neighborhood of which, would gain the maximum sensitivity. Thus, it can be expected TIRE signal would get the best response around SPR angle.



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Instrumentation

Figure 2:Schematic illustration of the experimental setup of the homemade EC-TIRIE system. The reaction in the experimental cell is measured simultaneously by both an electrical system and an optical system.

The potential variation is applied by a VersaSTAT 3 electrochemical system (Princeton U.S.A). A

28 gg × 16 gg SF 10 glass slide with a 2 jg Cr adhesive under-layer and a 50 jg Au serves as both the sensing surface and the working electrode, a Ag/AgCl electrode as the reference and a Pt wire electrode as the counter. Prior to each experiment, the chip is cleaned with ultra-pure water and pure ethanol to remove the possible surface contamination and then blown to dry.

Figure 2 illustrates the EC-TIRE setup for the experiments in this paper. A SF10 trapezoidal prism is used with a Xe lamp as the light source and a high-speed cool CCD camera as the detector. After the monochromator, 632.8 jg light beam is guided by an optical fiber and expanded by a

collimating system. After passing a polarizer and a compensator, the polarized collimated beam propagates perpendicularly to the prism and onto the sensing surface. When the incident angle is

57.5° , in the neighborhood of SPR angle for the system, the evanescent wave appears sharply at the sensing surface to detect the interaction in very shallow depth from the surface. The reflected light carrying the surface information is then imaged by a CCD camera after passing an analyzer. To achieve the best sensitivity, the system works under the null and off-null mode. By adjusting the polarizer and the analyzer, the null condition is achieved when no potential variation occurs on the sensing surface and when the potential on the surface changes, the system will detect the dielectric constant variations under the off-null condition. During the measurement, the optical signal variation from the potential modulation is recorded by CCD in grayscale.

Experimental Sodium sulfate (Na2SO4) is purchased from Sigma-Aldrich, which is in analytical grade without further purification. To prepare all the solutions, a MILLI-Q purification system is used to get the distilled water (18.2 at the room temperature.)

Since EDL structure of the electrolyte side is not only influenced by the potential variation but by the concentration of the electrolyte, three different concentration Na2SO4 solutions have been

prepared for the experiment: 1 g, 10 g and 100 g and compared with the distilled water.

All the electrolytes have been pumped into the cell at the speed of 10 e/gj.

For our applications 23,25, two kinds of potential modulation have been applied to the system. One is the conventional cyclic voltammetry (CV), the potential of which is driven by external

electrochemical station, linearly scanned from −0.2 * to 0.2 * at the scan rate 10 g*/. The other is open circuit measurement. For each electrolyte, charging the sensing surface with a 1

pulse of −0.25 * and measuring its open circuit voltage for 60 , then, we apply another 60

constant potential (from −0.2 * to 0.2 *, step 0.1 *) to the surface.

Results and Discussion


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Figure 3: (a) Linear sweep diagram, the potential of which is driven by external electrochemical station, linearly scanned from −0.2 * to 0.2 * at the scan rate 10 g*/; (b) during the scan, TIRE signal is recorded synchronously.

Figure 4: (a) Potential during the open circuit measurement; (b) synchronous TIRE signal.

Fig. 3 is for linear sweep procedure. As is seen in fig. 2, during 0 ~ 0.2 *, the reaction is a typical

non-faradaic charging/discharging process, while during −0.2 * ∼ 0 *, the current wave is due to the dissolved oxygen reduction that, according with the Nernst law, occurs in this potential region for neutral solutions with the formation of hydrogen peroxide, while the proton reduction requires electric potentials lower than -0.4V. In fig. 3(b), as predicted by eq. (15), all the TIRE signals bear the resemblance to the potential, which is linearly driven by the external electrochemistry station. A parallel shift among the optical curves can be attributed to the fact that the different concentrations of the electrolyte will give rise to the different unperturbed ellipsometric parameters of the surface under the specific polarization setup. Besides, the electrolyte concentration will also modulate TIRE response because EDLs of the solution are varied for different concentrations of electrolyte. This can be observed by the fractional change of TIRE response, D. − lD. /lD. . For the distilled water, the fractional change is 1.5%, 1 mM Na2SO4 2.2%, 10 mM Na2SO4 2.5% and 100 mM Na2SO4, 3.1%. Both the refractive index and the length of EDL exert influences on TIRE response. The electrolyte of higher concentration gives rise to more compact and thinner EDL, which indicates the EDL of the concentrated electrolyte has less thickness and large refractive index variation. That’s might be the reason why the fractional change of the dilute electrolyte is less than that of the concentrated electrolyte. Fig. 4 shows the optical signal variation under open potential circuit. Unlike linear sweep voltammetry, the potential of which is driven by the external electrochemistry station, the potential variation comes from the ion rearrangement of the EDL. Charged by the pulse, EDL would change near the electrode. Under the open potential condition, the ions in the EDL will rearrange positions and a new order is established. As is discussed in fig. 3(b), a parallel shift occurs because of the electrolyte concentration difference. In the electrolyte, the charge density change )( is given by

)( = −) = 0 5 ∑ L9 :9

( 17 )

where 0 is the thickness of the diffuse layer, which is usually less than the decay length of the

evanescent wave, Avogadro constant, L9 the (signed) charge on ion i and :9 the concentration change of ion i in the diffuse layer. Since TIRE signal detects the dielectric constant near the electrode, which is also the function of ion concentration by eq. (4). The optical signal and the open circuit are alike, which is obvious for 1 mM Na2SO4 in fig. 3. When flow disturbance occurs, both potential and optical signal varies similarly. In our estimation, for 0.1 V potential variation, the

normalized optical signal change of TIRE biosensor should be around 1% (where ) ≈ 0.001,

- ≈ 0.001), which is agreed with figs. 3 and 4. All the normalized optical responses (⁄) are at

the order of 1%.

Conclusion



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In this paper, we use EC-TIRE to observe the potential modulation on the sensing surface. The analysis suggests that both dielectric constant variation of gold and EDL transformation would modulate the reflection polarization of the surface. For a non-faradaic process, the optical signal of

TIRE would be proportional to the potential modulation and the variation should be around 1% for 0.1 * potential variation, which cannot be ignored in biosensor applications, especially for the

small scale biomolecule detection. Therefore, to use EC-TIRE for biomolecule detection, a reference cell should be introduced. By simulating the potential condition of the working cell, the reference cell would tell the optical variation introduced by the interfacial potential modulation. This signal should be ruled out from the biomolecule detection.

Acknowlegement The authors thank the financial support to the International Science & Technology Cooperation Program of China (2015DFG32390), to the 7th Sino-Portugal Scientific and Technological Cooperation of 2013-2015, to the National Basic Research Program of China (2015CB352100), to the National Natural Science Foundation of China (21305147) and (81472941) and to Portuguese Foundation for Science and Technology, FCT (projects: IF/00808/2013/CP1159/CT0003; UID/MULTI/00612/2013 ), POPH and UE, FSE.

SUPPORTING INFORMATION. The deduction of eqs. (1) and (9).

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For Table of Contents Only


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figure 1 (a) 846x635mm (72 x 72 DPI)



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Figure 2:Schematic illustration of the experimental setup of the homemade EC-TIRIE system. The reaction in the experimental cell is measured simultaneously by both an electrical system and an optical system. 213x157mm (300 x 300 DPI)



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figure 3 (a) 846x635mm (72 x 72 DPI)


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figure 3 (b) 846x635mm (72 x 72 DPI)



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figure 4 (a) 846x635mm (72 x 72 DPI)


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figure 4 (b) 846x635mm (72 x 72 DPI)


Potential Modulation on Total Internal Reflection Ellipsometry

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