construction of simultaneous spr and qcm sensing platform

ORIGINAL PAPER

Construction of simultaneous SPR and QCM sensing platform

JongMin Kim Æ SeongHoon Kim Æ Tatsuya Ohashi ÆHiroshi Muramatsu Æ Sang-Mok Chang ÆWoo-Sik Kim

Received: 17 May 2009 / Accepted: 5 August 2009 / Published online: 26 August 2009

� Springer-Verlag 2009

Abstract To construct a novel simultaneous SPR and

QCM sensing system, an AT-cut quartz crystal is fabri-

cated by sputtering 250 nm of ITO on one side of the

quartz plate over a 5-nm thick underlay of titanium, while a

50-nm thick layer of gold is sputter-deposited on the other

side to induce a total light reflection of an incident laser

beam on the thin gold layer. The signals of the sensing

system are detected using a Handy-SPR and QCA922 when

dropping 200 lL of Milli-Q water into the sensing cell.

A decrease in the SPR reflected light intensity is clearly

identified. In the same experiment, the changes in the

resonant frequency and resistance are about 2 kHz and

200 X, respectively. Furthermore, the oscillation stabilities

of the resonant frequency and resistance are about 50 Hz

and 2 X, respectively, which are sufficient to detect a large

mass change on the QCM/SPR chip.

Keywords SPR � QCM � Resonant frequency �Resonant resistance � SPR angle � ITO � Titanium �Index matching oil

Introduction

Chemical sensing systems are widely used in the fields of

health care, environmental monitoring and preservation,

and the agricultural and chemical industries. However, in

the real world, chemicals are often mixed and the con-

centrations of the target chemicals are invariably low,

thereby requiring complicated procedures and a long

examination time for the accurate detection of such

chemicals. Thus, a simple sensing device for detecting

target chemicals in chemical mixtures need to be devel-

oped, especially for use in health care, environmental

preservation, and process control. Several research groups

have already tested different combinations of receptors and

transducers. Yet, the selectivity and sensitivity are not

enough and the reliability is questionable, as the sensing

signal is usually obtained based on one analytical variable

only. Therefore, attempts have also been made to develop

sensing systems that obtain multiple sets of analytical

variables simultaneously.

The current authors previously developed simultaneous

sensing systems to analyze the viscoelastic characteristics

of polymer blend thin films using the quartz crystal reso-

nance (QCR) and differential scanning calorimetry (DSC)

[1], the local viscoelasticity and surface morphology of

polystyrene thin films using the QCR and atomic force

microscopy (AFM) [2], the electrochromic and viscoelastic

property changes of polypyrrole thin films during the redox

process using the QCR and UV–visible spectroscopy [3],

the capacitor characteristics and surface morphology of

polymer thin films using the QCR and AFM [4, 5], and the

morphology and physical properties of cultured cells using

a micro camera and quartz crystal [6, 7].

Furthermore, simultaneous sensing systems combining

the QCR and surface plasmon resonance (SPR) have

J. Kim � S. Kim � S.-M. Chang

Department of Chemical Engineering, Dong-A University,

840 Hadan-Dong, Saha-Gu, Pusan 604-714, Korea

T. Ohashi � H. Muramatsu

Department of Bioscience and Biotechnology,

Tokyo University of Technology, 1404-1, Katakura, Hachioji,

Tokyo 192-0982, Japan

W.-S. Kim (&)

Department of Chemical Engineering, ILRI,

Kyunghee University, Seochun 1, Kiheung, Yongin,

Kyungki-Do 449-701, Korea

e-mail: [email protected]

123

Bioprocess Biosyst Eng (2010) 33:39–45

DOI 10.1007/s00449-009-0370-5

recently been explored to improve the sensitivity and

reliability of both sensor systems [8–12]. In these appli-

cations, two types of response based on the piezoelectricity

and surface plasmons are used as the main information.

Piezoelectricity is a phenomenon related to the pro-

duction of an electrical potential when certain materials

(mainly crystals) are compressed. This process is also

reversible, as when an electric potential is applied to such

materials this produces deformation. Therefore, when an

alternating electric potential is applied to a piezoelectric

material, this generates an acoustic wave that is affected by

changes in temperature, pressure, and most importantly, by

changes in the physical properties at the interface between

the material surface and a foreign fluid or solid. As such,

piezoelectric crystal sensors are passive solid-state elec-

tronic devices that can respond to such changes. A selec-

tive sensor is then obtained when the sensor surface is

coated with a selectively interacting thin film, which was

introduced by King in 1964 based on a piezoelectric

sorption detector. A quartz crystal microbalance (QCM) is

also well known as a sensitive mass detecting device and

thin film microrheology monitoring device, and its theory

and applications have also been well documented in pre-

vious reviews [13]. In the case of a 9 MHz quartz crystal,

the mass sensitivity is normally of a nano gram order, but

applying a high frequency mode [14, 15] and an overtone

mode [16–18] produces better sensitivity within a sub-nano

gram order.

Meanwhile, surface plasmons are longitudinal charges

that propagate waves along the interface of a metal and a

dielectric. Surface plasmon can be excited using the

experimental systems developed by Otto and Kretchmann,

yet most SPR instruments use Kretchmann’s method,

which is an attenuated total reflectance (ATR) configura-

tion. Since the SPR occurs at the boundary of a metal and

external medium, these oscillations are very sensitive to

any change in the boundary, such as the adsorption of

molecules onto the metal surface or changes in the mor-

phology and structure. Its theory and applications have

already been well described by Rebecca et al. [19]. The

SPR response angle is related to the refractive index of a

material which is proportional to the mass change of the

target material. While the resonant frequency change of a

QCM is also related to the mass change of the target

material, it is additionally affected by the viscoelasticity of

the target material. Therefore, these complementary char-

acteristics mean that simultaneous detection using both the

SPR and a QCM makes the sensing system more accurate

and reliable by avoiding the individual limitations of each

sensor system. Thus, various studies have attempted to

analyze the sensing responses of materials using a con-

secutive combination of the SPR and a QCM. The direct

mass sensitivity comparison between QCM and SPR is

reported elsewhere [20]. Accordingly, this study fabricated

a QCM system that allows a simultaneous QCM and SPR

investigation. The responses of the SPR angle, resonant

frequency, and resonant resistance are all simultaneously

measured using various index matching oils, plus the fea-

sibility of practical applications is also examined.

Experiment

Materials and instruments

A 9 MHz AT-cut quartz plate (7.9 9 7.9 9 0.2 mm, QA-

A9M-AU, SEIKO E.G.&G), Ag paste (Dotite FA-705BN,

Fujikura Kasei), immersion oil(nd = 1.516, Olympus),

silicon oil (KF-56, Shinetsu Chemical), cover glass (No.1S

(18 9 18 9 0.18 mm), Matsunami), and Milli-Q water

(Milli-Q integral, Millipore) were all used to fabricate the

QCM/SPR sensing system. Plus, a batch-type sputter (CFS-

4ES-232, Shibaura), temperature-controlled incubator

(CN-25A, Mitsubishi), quartz crystal oscillation measuring

system (QCA922, SEIKO E.G.&G), probe-type film

thickness measurement instrument (Dektak 8, Veeco Co.

Inc.), and Handy-SPR (PS-0109, NTT-AT) were also used

as fabricating and measuring instruments.

Fabrication of simultaneous QCM/SPR chip

The choice of the electrode material is important to provide

a sharp SPR peak and good stability. The index matching

oil is also important to remove any dissonance between the

quartz plate and the bottom of the hemispherical prism

when the quartz crystal is oscillated, and should be a low

viscous medium to reduce the oscillating energy of the

quartz crystal. Furthermore, the index matching oil allows

an optical coupling of the quartz plate to the bottom of the

hemispherical prism to eliminate any gap between their

refractive indices [19].

One side of the AT-cut quartz crystal plate was sputtered

with 250 nm of ITO over a 5-nm thick underlay of tita-

nium, while the other side of the plate was sputtered with

50 nm of gold. The original diameter of the quartz crystal

electrode was 5 mm, and the thickness of the sputtered

electrodes was evaluated using a thickness measuring

instrument (Dektak 8, Veeco Co. Inc.). After sputtering, the

ITO electrode was annealed at 350 �C for 2 h to reduce any

inherent resonant resistance, and the final resonant resis-

tance was measured to be about 1.5 9 10-4 X cm. The

quartz crystal was conductively connected to a cover glass

with a gold surface and sealed using the same cover glass.

For this purpose, both sides of the cover glass were sput-

tered with 120 nm of gold over a 20-nm thick underlay of

chromium to transmit the QCM signals with a sputtering

40 Bioprocess Biosyst Eng (2010) 33:39–45

123

area of 7 9 7 mm2. Figure 1 shows a schematic illustration

of the preparation procedures for the novel simultaneous

QCM/SPR chip. The quartz crystal was optically coupled

to the prism, and any air removed by inserting the index

matching oil between the quartz crystal and the cover glass.

Novel cell assembly for simultaneous QCM/SPR

system

Figure 2 shows a schematic diagram of the principle of the

SPR system (a) and the proposed measuring cell (b) for the

simultaneous SPR and QCM sensing system. Normally,

SPR uses an irradiation light source of a 65–75� incidence

angle to produce evanescent wave on the surface of a gold

chip. Then, this evanescent wave and a surface plasmon

wave occurring on the surface cause a surface plasmon

resonance. As a consequence, the total reflected light

power will be particularly decreased at the incident angle.

This incident angle is called as the SPR angle, thus the

change of the refractive index caused by a certain kind of

mass loading is useful as a sensing response. Our purpose

is replacing the SPR chip with a QCM to enable multi-

functional sensing or analytical system. To enable the

purpose, a QCM should be oscillated in the SPR environ-

ment, and a purpose specific cell design is required. Thus,

only one reflection site of the QCM is necessary for

operating the SPR, but QCM requires both the electrodes

on both the quartz surfaces to oscillate. Thus, we used a

transparent metal electrode (ITO) as shown in Fig. 2b and

Fig. 1. In Fig. 2b, only one side of the quartz crystal

electrode was exposed to the liquid sample, while the other

electrode was mounted onto the bottom of a hemispherical

prism. The cell was filled with the index matching oil to

eliminate any air between the quartz crystal and the cover

glass. The cell was made of an acrylic plate and the height

controlled using a soft lithography technique with a PDMS

(polydimethysiloxane) stamp. The SPR instrument uses a

software provided by the manufacturer (Sprwin 3 for

Handy-SPR PS-0109, NTT-AT). The software supports a

continuous measuring mode with a gate time 3 s. In the

case of the QCM instrument, the possible gate time is about

0.1 s with a frequency resolution of 0.1 Hz and a resistance

resolution of 0.1 X. Thus, continuous measuring of both

the systems can be possible with a gate time 3 s.

Results and discussion

SPR response of QCM/SPR chip

The test cover glass used was No.1S (Matsunami, thick-

ness = 0.18 mm), as its thickness was similar to the

thickness of the quartz crystal. The glass was prepared by

ultrasonic cleaning in acetone for 5 min and Milli-Q water

for another 5 min, then dried using a vacuum pump. A gold

electrode was then fabricated on the cover glass using a

sputtering technique. Thereafter, 5 ll of the immersion oil

(nd = 1.516), as the index matching oil, was dropped on

the bottom of the hemispherical prism, and the prepared

cover glass placed over the top. The SPR response

according to the function of the gold layer thickness was

then investigated using SPR instruments, and the maximum

resonant peak obtained with a 50 nm gold layer. As the

adhesion of pure gold to the cover glass was not strong

enough to be used as a measuring device, the gold was

sputtered over another adhesive metal layer with a strong

adhesion to the cover glass to prevent the gold layer

peeling off the cover glass. A titanium film was used as the

adhesive metal layer over the cover glass, and the effect of

the titanium film on the SPR response was examined.

An adhesive metal film of one tenth the thickness of the

target electrode was found to be sufficient. Thus, a 50-nm

gold layer was sputtered over a 5-nm titanium layer. The

SPR response was then measured by dropping 100 ll of

Milli-Q water on the gold layer and the response compared

with that of a standard commercial SPR chip (Gold chip

011992, NTT-AT). The results are presented in Fig. 3.

While the resonant angle of the proposed chip was about 2�

Fig. 1 Fabrication procedure

for a simultaneous QCM/SPR

chip. The electrode construction

is based on 3-metal layers to

enable both the QCM and the

SPR function. a Preparing cover

glass, b Fixing QCM, c Final

QCM/SPR chip

Bioprocess Biosyst Eng (2010) 33:39–45 41

123

different from that of the commercial chip, the sharpness of

the SPR response was almost the same. The difference in

the resonant angle may have originated from the refractive

index gap between the two kinds of metal electrode. Thus,

the results confirmed that the proposed SPR chip can be

used as a measuring chip in the new QCM/SPR system.

QCM stability tests with QCM/SPR system under

atmospheric conditions

The stability of the QCM in the simultaneous sensing

system was first tested in an incubator at 25 �C. The res-

onant frequency and resonant resistance were measured

using a QCA922, and the results are presented in Fig. 4.

After 10 min, the signal responses reached a steady state,

while the deviations of the resonant frequency and resonant

resistance were about 5 Hz and 1 X in the stable area,

respectively, thereby confirming that an AT-cut quartz

crystal fixed with cover glasses can be used as a simulta-

neous sensing chip under atmospheric conditions.

Effect of matching oil on QCM response

The stability of the QCM in the simultaneous sensing

system was examined after injecting an index matching oil,

such as immersion oil (nd = 1.516, Olympus Co.) or sili-

con oil (KF-56, Shinetsu Chemical Co.), between the

quartz crystal and the cover glass. Figure 5 shows the

results obtained when using the high viscous immersion oil

as the index matching oil, while Fig. 6 shows the results

when using the low viscous silicon oil.

When the high viscous immersion oil was injected, the

resonant frequency decreased to about 17 kHz when

compared with the oscillation frequency in air (compare

Figs. 4 and 5). After 30 min, the response of the resonant

frequency reached a stable area with a frequency deviation

of about 300 Hz. Meanwhile, the resonant resistance

Fig. 2 Schematic illustration of

SPR principle (a) and proposed

measuring cell (b). The

proposed measuring cell is

fabricated to fulfill the

requirement of both the SPR

and the QCM operation

Fig. 3 SPR responses of standard SPR chip (A) and proposed SPR

chip (B) under atmospheric conditions Fig. 4 Typical resonant frequency response (A) and resonant resis-

tance response (B) of QCM in simultaneous sensing system under

atmospheric conditions

Fig. 5 Typical resonant frequency response (A) and resonant resis-

tance response (B) of QCM in simultaneous sensing system after

injecting immersion oil (nd = 1.516) between quartz crystal and

cover glass


123

increased about 10-fold when compared with the resonant

resistance value in air (compare Figs. 4 and 5). In the stable

area, the deviation was about 10 X, as shown in Fig. 5.

When the low viscous silicon oil was injected, the res-

onant frequency decreased to 6.8 kHz when compared with

the oscillation frequency in air, as shown in Fig. 6. After

30 min, it reached a stable area with a frequency deviation

of about 10 Hz. The resonant resistance increased about

4-fold when compared with the resonant resistance value in

air, while the deviation was about 1 X in the stable area,

which was similar to that in air.

Therefore, the results revealed that a low viscous

matching oil was more effective for the proposed QCM/

SPR simultaneous sensing system.

Effect of proposed sensing cell on resonant response

of QCM

The influence of the proposed QCM/SPR cell on the

resonant oscillation properties of the QCM was measured.

In the designed cell, a screw and O-ring were used to

enable just one side of the quartz crystal electrodes to be

exposed to the liquid samples. The influence was then

measured in an incubator at 25 �C after injecting 5 ll of

silicon oil into the cell as the index matching oil. The

measured results are presented in Fig. 7, and show that the

deviations of the resonant frequency and resonant resis-

tance were about 20 Hz and 2 X after reaching a stable

area, respectively. These deviations were about 2-fold

higher than those in the incubator without the measuring

cell, thereby confirming the effectiveness of the proposed

sensing cell.

Water dropping experiments for obtaining simultaneous

QCM/SPR signals

Finally, the efficacy of using an At-quartz crystal for the

proposed SPR/QCM simultaneous sensing system was

examined by measuring the SPR, resonant frequency, and

resonant resistance using a Handy-SPR and QCA922

simultaneously when dropping 200 ll of Milli-Q water on

the gold electrode. The results are presented in Fig. 8.

The reflected light intensity was observed to change,

where the sharpness of the resonant angle was about 85%

of that of the cover glass sputtered with a 50-nm gold layer,

as shown in Figs. 3 and 8. The choice of the electrode

material is considered to be important to provide a sharp

SPR resonance peak. In Fig. 8, the QCM was sputtered

with 250 nm of ITO on one side of the quartz plate over a

5-nm thick underlay of titanium, and a 50-nm gold film on

the other side. This probably induced the reflection of light



injecting silicon oil between quartz crystal and cover glass



fixing proposed measuring cell

Fig. 8 Simultaneous sensing responses of a SPR and b QCM when

dropping water. In panel b, (A) denotes resonant frequency response,

and (B) denotes resonant resistance response


123

not only from the gold layer but also from the ITO or

titanium layer, thereby weakening the intensity of the light

that reached the gold layer and resulting in broad SPR

peaks. Thus, if the thickness of the sputtered metal layer

can be controlled more precisely to be thinner, the sharp-

ness of SPR peak could be improved for use as a sensing

kit.

The dropping of the water produced a decrease in the

resonant frequency and increase in the resonant resistance

of about 2 kHz and 100 X (compare Figs. 7 and 8),

respectively. These changes are within the normal range

that occurs when a QCM comes into contact with water

[13]. However, the deviations in the resonant frequency

and resonant resistance were only about 50 Hz and 2 Xafter reaching a stable area (time after 1 h), respectively,

which are not enough to detect a small mass change and

only effective for detecting a large mass change, such as

living cells under certain conditions [6, 7]. The increase in

the response deviation of the QCM was mainly caused by

the viscosity of the index matching oil. Thus, the problem

of signal noise can be solved if a lower viscous index

matching oil is used in the future. Plus, further develop-

ment is required applying a high frequency [14, 15] or

overtone [16–18] modes of the QCM to enhance the sen-

sitivity. Various applications, which are normally treated as

a possible area of QCM application, may be possible in

biosensor with these developments [13]. Finally, the data is

reproducible within the standard deviation range of 5% in

our 5 experiments continuously with a similar response

shape.

Conclusion

This study fabricated and investigated a QCM for a

simultaneous SPR and QCM sensing system. In a water

dropping experiment, the deviations in the resonant fre-

quency and resonant resistance were about 50 Hz and 2 X,

respectively, after reaching a stable area, which are not

enough to detect a small mass change. Meanwhile, the

sharpness of the SPR angle was shown to depend on the

kind of electrode material used and its thickness, and the

response stability of the QCM to depend on the viscosity of

the index matching oil. As such, a more sensitive mea-

suring system can be designed by selecting an adequate

electrode material combination and lower viscous match-

ing oil. Therefore, the results confirmed the possibility of

developing a simultaneous SPR/QCM sensing platform.

Acknowledgment This research was supported by Basic Science

Research Program through the National Research Foundation of

Korea (NRF) funded by the Ministry of Education, Science and

Technology (KOSEF, 2009-0064245).

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construction of simultaneous spr and qcm sensing platform

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