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Title

Surface proton transport of fully protonatedpoly(aspartic acid) thin films on quartz
substrates

Author(s)Nagao, Yuki; Kubo, Takahiro

Citation

Applied Surface Science, 323: 19‑24

Issue Date

2014‑06‑19

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/12335

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Yuki Nagao,
Takahiro Kubo, Applied Surface Science, 323,
2014, 19‑24,
http://dx.doi.org/10.1016/j.apsusc.2014.06.085

Description

Japan Advanced Institute of Science and Technology

Surface proton transport offully protonated poly(aspartic acid) thin films on quartz substrates

Yuki Nagao*, Takahiro Kubo

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923-1292, Japan
Corresponding author: ynagao@jaist.ac.jp, +81(Japan)-761-51-1541

Abstract
Thin film structure and the proton transport property of fully protonated poly(asparticacid) (P-Asp100) have been investigated. An earlier study assessed partially protonatedpoly(aspartic acid), highly oriented thin film structure and enhancement of the internal
proton transport. In this study of P-Asp100, IR p-polarized multiple-angle incidence
resolution (P-MAIR) spectra were measured to investigate the thin film structure. The
obtained thin films, with thicknesses of 120 nm to 670 nm, had no oriented structure.
Relative humidity dependence of the resistance, proton conductivity, and normalized
resistance were examined to ascertain the proton transport property of P-Asp100 thin

1

films. The obtained data showed that the proton transport of P-Asp100 thin films mightoccur on the surface, not inside of the thin film. This phenomenon might be related withthe proton transport of the biological system.

Keywords
Proton conduction, Surface conduction, Polypeptide, Bacteriorhodopsin

2

1. Introduction
In biological systems, proton transfer in protein ionic channels has attracted many
researchers since the 1960s [1]. Many have investigated the purple membrane including
bacteriorhodopsin for its fundamental role in proton transport in biological systems[2–11]. Surface proton conduction at the lipid monolayer has also been studied
intensively [12-18]. Reportedly, 20-times enhancement of proton diffusion on thesurface was achieved compared to bulk water [12], but other researchers strongly
disputed the contents of that report [13]. Nevertheless, enhancement of ionic transport at
the interface still attracts many researchers of surface science and ionics.
Regarding work in the solid state ionics research field, Liang reported
approximately 50-times enhancement of Li+ ion conduction in 1973 [19]. The sample
was simply mixed with inert Al2O3 ultrafine particles into LiI. However, ionic
conductivity depends strongly on the contents of Al2O3 ultrafine particles. In most cases,
when enhancement was observed, the second phase consisted of a nonconducting
material such as SiO2 or Al2O3, which is nearly insoluble in the host material. Thisfinding gave us the opportunity to improve the ionic transport property by the
inorganic–organic hybrid system. Recently many hybrid materials have been reported
for proton conductors with consideration of the pKa difference between the proton3

donor and acceptor.
Many acid–base combinations of proton conductors have been reported for
improving proton transport and stability or for anhydrous proton conduction [20–23].
The pKa difference between the acidic and basic property of materials was applied forthe proton transport system based on the concept of a proton donor and acceptor.
Another familiar proton transfer with the pKa difference is apparent in the biologicalsystem: bacteriorhodopsin. It pumps protons from the cell interior to the external
medium when the light is irradiated. The retinal in bacteriorhodopsin receives light for
the activation. Then it changes the pKa-correspondent amino acid residues step-by-steparound the retinal. The aspartic acid residue in bacteriorhodopsin polypeptides plays acrucial role in proton transport [2,7].
Recently, we have found markedly high proton conductivity in the thin films ofpartially protonated poly(aspartic acid) on quartz substrates [24]. This thin film
exhibited highly molecular orientation spontaneously. The wavenumber of the IR
spectrum suggests that this thin film has non-periodic secondary structures, e.g., β-turn[25] and α-sheet [26] structures. The proton conductivity of the thin film was one orderof magnitude higher than the random oriented specimen. Results suggest that the origin
for the proton conductivity enhancement is attributable to the oriented structural effects.

4However, a study of this enhancement of the proton transport using poly(aspartic acid)itself is insufficient to understand this phenomenon. In this study, fully protonatedpoly(aspartic acid) (P-Asp100) was synthesized to investigate the proton transport
properties of thin films. Although P-Asp100 has movable protons at the side chain,
protons are not easily released and are hard to move to the next proton site in P-Asp100
because of the weak acid pKa and fully protonated state. Therefore we speculated thatthe results obtained in the case of P-Asp100 would differ from those of the partially
protonated poly(aspartic acid).

2. Experimental
Fully protonated P-Asp100 was synthesized according to Scheme 1. D, L-aspartic
acid, o-phosphoric acid, methanol, sodium hydroxide, and HCl were purchased from
Wako Pure Chemical Industries Ltd. D, L-Aspartic acid of 5 g and 85% o-phosphoricacid of 3 ml were added to a 100 ml round-bottom flask and were mixed for 10 min
under Ar atmosphere. The mixture was heated at 170°C for 16 hr. The product waswashed with methanol and water. The residue was washed with methanol and dried
under reduced pressure. The dried sample of 3.6 g and a solution of sodium hydroxideof 1.5 g with 15 ml of deionized water were added to a 200 ml beaker with stirring for5

20 min. A white precipitate was obtained by adding methanol of 100 ml. The sample
was washed with methanol and was separated from the supernatant, in which thecentrifuge technique was used in washing and separation procedures. The obtained
sample was dried under reduced pressure. For protonation, the sample was dissolved
completely in distilled water. Then an ion exchange process was performed using
Amberlyst. Using a rotary vacuum evaporator, the protonated sample was obtained anddried under vacuum. In fact, 100% of protonation was confirmed using the titration
method. The obtained sample (P-Asp100) was characterized via 1H NMR (Avance III;
Bruker BioSpin Inc.) and FT-IR measurements using an ATR accessory (Nicolet 6700;
Thermo Fisher Scientific Inc.). Gel permeation chromatography (ChromNAV-GPC;
Jasco Corp.) showed the average molecular weight as ca. 6500, as determined.
The P-Asp100 thin films were prepared by spin coating using a spincoater(ACT-200; Active) on mirror-polished quartz substrates or Si wafer. The quartz
substrate and Si wafer size were, respectively, 17 mm  1 mm and 20  30  0.5 mm.
Before film deposition, each substrate was subjected to several cleaning processes. Then
plasma treatment was performed using a vacuum plasma system (Cute-MP; Femto
Science) to improve surface hydrophilicity. The thickness was determined using acontact stylus profiler (P-10; KLA Tencor Corp.). The roughness measurement showed

6that its smoothness was sufficient for IR measurement and impedance analysis.
For p-polarized multiple-angle incidence resolution (p-MAIR) spectroscopy[27–30], measurements were taken using a FT-IR spectrometer (Nicolet 6700; Thermo
Fisher Scientific Inc.) equipped with a mercury–cadmium–telluride (MCT) detector. A
mirror-polished Si wafer was used for p-MAIR measurements. Single-beam spectra
were collected from 38° to 8° at 6° steps between the angle of incidence. The aperture
was fully opened (size of 150) and a metal plate with small pores was placed in the lightpath of the incident beam to prevent saturation. To obtain p-polarized light, a ZnSepolarizer was used. The cleaned Si wafer was used to obtain reference data.
For impedance measurements, the thin films were measured in relative humidity(RH) of 70–90% using an impedance/gain-phase analyzer (SI1260; Solartron
Analytical) and a dielectric interface system (1296; Solartron Analytical). The RH andtemperature were controlled using a humidity-controlled and temperature-controlledchamber (SH-221; Espec Corp.). The temperature was set at 298 K. The thin films wereprepared by spincoating on the quartz substrate. The thin film size was ca. 5×10 mm.
The electrode configuration was selected to obtain measurements of the current flow inthe plane parallel to the substrate surface. The electrode was located at the edge of thefilm, with a parallel electrode configuration. Porous gold paint (SILBEST No. 8560;

7

Tokuriki Chemical Research) was used for electrodes and was covered at the side of thefilm. The inter-electrode distance was ca. 4.0 mm.

3. Results and Discussion
3.1 Characterization of P-Asp100
Figure 1 shows the 1H NMR spectrum for P-Asp100. DMSO-d6 was used for thesolvent. The peaks at 4.5 and 2.6 ppm are attributed respectively to the CH and CH2
groups. The peaks at 8.1 and 12.5 ppm respectively correspond to the protons of NH
and COOH groups. Scheme 1 shows that integration of proton peaks in 1H NMR
spectrum produces good agreement with the number of protons.
Figure 2 shows the ATR spectrum for P-Asp100. The amide I (CO of amide group)
and amide II (NH of amide group) bands appeared respectively at 1650 and 1530 cm-1.
The wavenumbers of these bands are almost identical values to those of the partially
protonated poly(aspartic acid) [24]. The new band at 1730 cm-1 corresponds to the CO
vibration mode of COOH at the side chain. Absorption around 1600 cm-1 derived fromthe deprotonated COO- group was not observed because P-Asp had no COO- group.

3.2 Thin film structure8In an earlier study of the partially protonated poly(aspartic acid) thin film, thinfilms were prepared on the MgO substrate [24]. In this study, the proton transport
property was investigated with the quartz substrates. Therefore for the measurement of
p-MAIRS, as-received Si wafer was used without removing the oxidized surface. The
obtained thickness of the thin films was 120–670 nm. Figure 3(a) shows p-MAIR
spectra for the 120 nm thick P-Asp100 thin film on the Si wafer. Absorption around1600 cm-1 derived from the deprotonated COO- group was not observed. This result is
consistent to obtain fully protonated P-Asp100. In the in-plane mode, the amide I andamide II bands were observed, respectively, at 1655 and 1530 cm-1. The in-plane
spectrum was similar with the ATR spectrum in Fig. 2. However, in the out-of-plane
spectrum, all peaks were shifted to the higher wavenumber. The amide I and amide II
bands in the out-of-plane were observed, respectively, at 1667 and 1539 cm-1. The COmode of the COOH groups was also shifted respectively from 1730 to 1742 cm-1. The
reason for these shifts has not been revealed. In a previous study with the partially
protonated poly(aspartic acid) thin film, the shapes of spectra between those in-planeand out-of-plane differed greatly. The amide groups had a highly oriented structure [24].
However, spectra with fully protonated P-Asp100 thin film were found to be similar
excluding the peak shift between the in-plane and out-of-plane spectra in this study. The

9degree of the orientation was calculated using the following eq. (1) [31].

  tan 1

2 AIP
AOP

(1)

Therein,  is the orientation angle from normal to the surface; AIP and AOP respectively
stand for band intensities of identical bands in the in-plane and out-of-plane spectra. The
respective degrees of orientation for the amide I and amide II bands were 52° and 56°.
These angles were close to the 54.7°, the so-called magic angle [32]. The amide I andamide II groups correspond to a disordered state. For COOH groups at the side chain,
the degree of the orientation was 48°. The value was lower than that of the magic angle.
This result suggests that the COOH group had an oriented state.
To confirm the thin film structure changes that occur with the thickness, Figs. 3(b),3(c), and 3(d) show p-MAIR spectra with thicknesses of 300, 590 and 670 nm. Alongwith the thickness, the band intensity of COOH group increased slightly compared tothat of amide I. Table 1 presents the wavenumber for each in-plane and out-of-planeband. No thickness dependence was observed in this thickness range, but allwavenumbers of the out-of-plane spectrum were shifted to a higher wavenumber thanthose of the in-plane spectrum. Table 2 presents the degree of orientation for eachassigned band calculated using eq. (1). As a trend, the degree of the orientation
decreased with thickness, but no large structural change was observed for 120–670 nm
10thickness. The thin film structure does not change drastically with thickness in
P-Asp100.
Figure 4 shows the RH-dependent ATR measurements for the 120-nm-thick
P-Asp100 thin film. These measurements were conducted at 50%, 70%, and 90% RH
with the time interval of 30 min for every 20% RH increment. Before eachmeasurement, the sample was maintained at 298 K and under the specified humidity
condition for 30 min. Each measurement was performed within 10 s of the sample being
removed from the humidity chamber. The band intensity of the COOH group and amide
I increased slightly with RH. The thin film adsorbs water molecules with the RH.
Presumably, the adsorption of the water molecules can contribute to increased band
intensity of COOH group and amide I.

3.3 Proton transport property
Typical complex impedance plots of the P-Asp100 thin film with 600-nm-thick at
70% RH are presented in Fig. 5. The impedance was obtained from the semicircle
diameter. The obtained resistance was 6  1010 . Figure 6 shows the relative humidity
dependence of the resistance. The dependence was investigated using three thin films
with respective thicknesses of 100 nm, 320 nm, and 600 nm. The Log (resistance)

11

depended strongly on the relative humidity. Furthermore, the relation was linear. The
difference of the resistance between 70–90% RH was greater than three orders ofmagnitude. Presumably, this large difference was derived from the water uptake in thethin films. No thickness dependence of the resistance was observed. In the typical case,the resistance is expected to depend on the thickness. Therefore, the following two
methods were considered for the discussion. One is an internal proton transport in
P-Asp100 thin film as proton conduction. Another is a surface proton transport assurface conduction. Thin-film conductivity (σ) was estimated as
 

d
Rlt

(2)

where d stands for the distance between the gold electrodes, R signifies the resistancevalue obtained directly from the impedance measurement, l denotes the length of thecontact electrodes, and t represents the film thickness. Figure 7 shows the RH
dependence of the proton conductivity. The conductivity depends on both RH andthickness. The conductivity increased concomitantly with decreasing thickness. In the
typical case, the conductivity is independent of the thickness. When the conductivity
depends on the thickness, one must consider the surface effect for enhanced conduction[33–36]. Therefore the normalized resistance (R’) was also calculated using thefollowing equation to investigate the possibility of the surface conduction.

12

R' 

Rl
d

(3)

Figure 8 shows the RH dependence of the normalized resistance. The normalized
resistance did not depend on the thickness. It gave an almost constant value. If theinternal conduction occurred in the thin film, then the normalized resistance depends on
the thickness. It is therefore likely that the P-Asp100 thin film has only surface
conduction. Few reports describe the surface proton transport in synthesized polymer
thin films. In a previous study, the partially protonated poly(aspartic acid) exhibitedinternal proton conduction [24]. As one possibility explaining the reason for surface
transport, the structure was completely different between P-Asp100 and the partially
protonated poly(aspartic acid) thin films. In fact, the partially protonated P-Asp thinfilm had an oriented structure. This oriented structure was more ordered than that of
P-Asp100 thin film. A proton might be mobile into the inside of the thin film. However,the P-Asp100 thin film was not oriented. Therefore, protons might have difficulty
moving to the inside of the thin film rather than to the surface. Another contribution forthe surface transport is derived from the weak acid pKa. Protons exist at the COOH
groups of the side chain. This proton exhibits a weak acid pKa. If the proton is fully
charged in P-Asp100 based on the weak acid property, then proton is not released easily
from the acidic site of the COOH group.

13In summary, the thin film structure of the fully protonated P-Asp100 was found todiffer greatly from that of the partially protonated poly(aspartic acid). In an earlier study,
the thin film of the partially protonated poly(aspartic acid) exhibited highly internal
proton transport. However, in this study, dominant proton transport would occur on thesurface of the thin film, not inside of the film. This difference of the proton transport
can derive from two reasons: the different structure and the characteristic property of
the weak acid pKa. In P-Asp100, the COOH groups play a role as the proton donor. The
system has no proton acceptor. It is necessary to consider the balance of pKa whentreating the weak acid system. This result could contribute to understand the protontransport in biological systems.

4. Conclusion
Fundamental proton transport in a purple membrane has been investigated since the1970s, the aspartic acid residue plays a crucial role in proton transport. The
conformation change of the aspartic acid residues provides an opportunity for changing
pKa. Therefore, it partially plays a role in pumping protons from the cell interior to theexternal medium proton pump. In our previous study of the partially protonated14

poly(aspartic acid), a highly oriented thin film structure and enhancement of the internal
proton transport were observed. In this study, to elucidate the proton transport propertyof the poly(aspartic acid) further, the proton transport property of the fully protonatedpoly(aspartic acid) (P-Asp100) was investigated. Judging from the obtained data, theproton transport of P-Asp100 thin films might occur on the surface, not inside of thethin film. This result differs greatly in the case of the partially protonated poly(asparticacid). This difference in proton transport can derive from the different structure or fromthe characteristic property of the weak acid pKa. In P-Asp100, the fully protonated
COOH groups play a role only as a proton donor, with no proton acceptor in the system.
Therefore protons can not be released easily with mobility into the thin film. It isnecessary to consider a balance of pKa when treating a weak acid system. This result is
expected to contribute to the elucidation of proton transport in biological systems.

Acknowledgments
This work was supported financially by the Japan Society for the Promotion of
Science (JSPS) through the Funding Program (GR060) for Next Generation
World-Leading Researchers (NEXT Program), initiated by the Council for Science andTechnology Policy (CSTP). This work was partially supported by the Ogasawara

15

Foundation for the Promotion of Science and Engineering.

16

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22

Scheme and Figure captions
Scheme 1 Synthesis of fully protonated poly(aspartic acid) (P-Asp100).
Figure 1 1H NMR spectrum for P-Asp100.
Figure 2 IR spectrum by ATR method for P-Asp100.
Figure 3 p-MAIR spectra for P-Asp thin films with different thicknesses: (a) 120 nm,(b) 300 nm, (c) 590 nm, and (d) 670 nm. The IP component has two orthogonal
directions of the electric field. Therefore, the IP absorbance was multiplied by afactor of 2 [27].
Figure 4 IR spectra with different humidity conditions by ATR methods. Dotted line isunder 50% RH. Dashed line is under 70% RH. Solid line is under 90% RH.
Figure 5 Typical impedance plots for P-Asp100 thin film with 600 nm thick under 70%
RH and 298 K.
Figure 6 Relative humidity dependence of resistivity for P-Asp100 thin films: □, 100
nm thick; ○, 320 nm thick; , 600 nm thick.
Figure 7 Relative humidity dependence of proton conductivity for P-Asp100 thin films:□, 100 nm thick; ○, 320 nm thick; , 600 nm thick. These are invalid data. See
3.3 Proton transport property.
Figure 8 Relative humidity dependence of the surface resistance for P-Asp100 thin23

films: □, 100 nm thick; ○, 320 nm thick; , 600 nm thick.

24

Table captions
Table 1 Summarized IP and OP wavenumber position of CO mode of COOH groups,
amide I and amide II with each thickness
Table 2 Summarized degree of orientation for the CO mode of COOH groups, amide I,
amide II, and CN mode with each thickness

25

Table 1

Thickness IPCOOH OPCOOH IPamideI OPamideI IPamideII OPamideII/ nm/ cm-1/ cm-1/ cm-1/ cm-1/ cm-1/ cm-1

120

1730

1742

1655

1667

1530

1539

300

1730

1740

1657

1667

1533

1536

590

1733

1739

1660

1667

1535

1538

670

1735

1740

1660

1665

1533

1539

26

Table 2

Thickness

COOHamideIamideIICN

120 nm

48°

52°

56°

57°

300 nm

47°

51°

54°

57°

590 nm

47°

49°

53°

56°

670 nm

47°

49°

52°

54°

27

Scheme 1
O

NaOH

OH

N

OH ▲

H2N
O

O H
C N

O

O

H+
exchangeC ONa
nD, L-aspartic acid Polysuccinimide
Asp
PSI

n
O
Sodium polyaspartate

28

O H
C N
C OH

n
O
Poly(aspartic acid)
P-Asp100

Fig. 129

Fig. 230

Fig. 331

Fig. 432

Fig. 533

Fig. 634

Fig. 735

Fig. 836