Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA03 / P15
Development of Laser Excitation Thermal Lens Spectroscopyby Deep Ultraviolet Excitation (LETLS-213)
Aira KAMITO*, Miki ISODA, Akira HARATA
Department of Molecular and Material Sciences, IGSES, Kyushu University, Kasuga city, Fukuoka, Japan
Email: kamito.aira.753@s.kyushu-u.ac.jp
Abstract: A laser excitation thermal lens spectroscope is developed by using a deep ultraviolet laser for highly sensitive
detection and microanalysis of amino acids. The analytical method of all kind molecules in solution without chemical
modification is strongly required for advance of biochemistry. Laser induced fluorescence method is major as a highly
sensitive analytical method of molecular in solution, but the method is limited to detect target by fluorescence.
Keywords: Deep ultraviolet, 213 nm, Micro-fluid device, Thermal lens spectroscopy, Amino acid
1. INTRODUCTION
Laser induced fluorescence (LIF) is the highly
sensitive analytical method for fluorescent molecules insolution [1]. Species of high fluorescence quantum yieldsuit LIF method, but low or non-fluorescent species needto derivatize a fluorescenct functional group and there isa possibility of changing physical properties by chemicalmodification. However, most biochemical species are
non-fluorescent and absorb light only in ultraviolet (UV)region (400 nm >𝜆). Especially, amino acid that is thesource of protein absorb light only in deep UV region(240 nm >𝜆).
This study is focused on highly sensitive detection
and microanalysis without chemical modification. Laser
excitation thermal lens spectroscopy [2-4] is developedusing by deep UV laser (𝜆"= 213 nm) (LETLS-213) todetect amino acids. For microanalysis, a sample cell isreplaced by a micro-fluid device that can pour these typesof solutions from three inlets at the same time to control
the width of flow pass, and the thermal lens (TL) signal
is detected. Experimental conditions are optimized byevaluating signal to background (S/B) ratio.

2.2 Experimental setup
Experimental setup of LETLS-213 is schematicallyshown in Fig. 2. The excitation beam that are emitted bya high repetition rate solid-state diode pumped Qswitched laser (Nd:YVO4 laser, 𝜆"= 213 nm, IMPRESS
213, Xiton Photonics) pass a chopper for intensity
modulation (chopping frequency f= 100 Hz). The
excitation beam combines with the probe beam (He-Nelaser, 𝜆&= 633 nm) by a dichroic mirror. Two beams passa pinhole (𝜑= 0.6 mm) and a condenser lens (focal lengthf213= 83.05 mm, f633= 97.56 mm), then two beams
irradiate the flowing sample in the micro-fluid device.
The target amino acid in middle layer absorbs theexcitation beam and excite to a high energy state. The
refractive index of target sample around excitation beamirradiation changes by TL effect [5]. Only the probe beampass through a band pass filter (transmission wavelength𝜆( = 632±20 nm), is detected with a photodiode (PD),and gives TL signal.

2. EXPERIMENTAL METHOD
2.1 Materials and micro-fluid device
The target amino acid used is L-tryptophan (opticalabsorption wavelength= 220 nm), the solvent is 80/20(v/v%) acetonitrile aqueous solution. Two kind ofsolutions forming three liquid layer flow into the microfluid device (flow pass width= 300 µm, depth= 100 µm,quartz) from each syringe (5 mL) as shown in Fig. 1.
Middle layer is sample solution (1.0 × 10-3 M Ltryptophan solution of the solvent), and both side of
middle layer are sheath solution (solvent).
For checking laminar flow in the micro-fluid device,rhodamine 110 chloride and rhodamine B is used as
fluorescent reagents.

Excitation
Nd:YVO4laser(213 nm)

Micro-fluid device
300 μmMirror

Chopper

Probe
Dichroic
mirrorPinholeCondenser
lensMicro-fluiddevice
Lock-in Amp.
Oscilloscope

Mirror

PDHe-Nelaser(633 nm)Syringepump[ Sheath ][ Sample ][ Sheath ]Band passfilterFig. 2. Experimental setup of LETLS-213.
2.3 Experimental conditions
2.3.a The dependence of middle layer width
The sample flow of middle layer and the sheath flowof both outer layers are changed to make the total flowrate of three layer in the micro-fluid device 0.3 mL/min.
The micro-fluid device is fixed 100 mm behind thecondenser lens, the photodiode is fixed 100 mm behindthe micro-fluid device. In the same flow condition,laminar flow is checked by a fluorescent microscope.

SheathSampleSheathFig. 1. The flow of solution in the micro-fluid device.

-5-2.3.b Dependence of distance between the condenser
lens and the micro-fluid device
The sample flow and the sheath flow per a layer are
0.1 mL/min. The condenser lens and the photodiode are
fixed on this experiment. The micro-fluid device ismoved between the condenser lens and the photodiode.

is behind of two focal points without beam loss at thedistance of 104 mm.
0.5

TL signal (mV)

0.4

3. RESULTS and DISCUSSION
3.1.a The dependence of middle layer width
In each experiment from (a) to (e), the flow velocity
in the micro-fluid device is constant by keeping the total
flow rate as a constant in table 1. As the middle layer
width is wide, the S/B ratio increase as in Fig. 3. The
result shows that the wider flow width, the more aminoacid is excited. Laminar flow is also confirmed for all
conditions of flow rates as shown in Fig. 4.
Table 1. Flow rate of each experiment.
Flow rate (mL/min)
SampleSheatha0.250
0.025
b0.2000.050
c0.1500.075
d0.1000.100
e0.0500.125
80
S/B ratio

60cd40

0.2
0.1
090

95

100

105

110

Distance between the condenser lens andthe micro-fluid device (mm)Fig. 5. The dependence of TL signal intensity on theposition of the micro-fluid device.
Distance

Total
0.300
0.300
0.300
0.300
0.300

b0.3

93 mm

98 mm

aProveExcitation

FilterSample

PDSample

Sample104 mmSample20e108 mm0Fig. 6. Schematic illustration of prove beam divergencechange by TL effect for four different positions of thesample. The exciting beam and the probe beam passthrough sample.

0100200300
Width of middle layer (sample) (µm)
Fig. 3. Dependence of S/B ratio on the middle layer width
in the micro- fluid devise.
(b)

4. SUMMARY
The setup of LETLS-213 is optimized for highly
sensitive detection and microanalysis without chemicalmodification. The micro-fluid device can control thewidth of the sample flow remaining laminar flow. Thewider is the width of middle layer, the stronger the TL
signal. The micro-fluid device has an optical position of104 nm for this system.

(d)

100 μm

100 μmFig. 4. Laminar flow in the micro fluid device under thecondition of (b) and (d).

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3.1.b Dependence of the TL signal intensity on theposition of the micro-fluid device
The strongest TL signal is obtained at the distance of104 mm as in Fig. 5. The position of the strongest TL
signal is behind the focal points of excitation beam andprobe beam as in Fig. 6. According to theory, strong TL
signal is obtained near the focal point of excitation beamin front of the focus of probe beam. However, this opticalsystem causes beam loss because the probe beam is toolarge, and the irradiation area is too narrow. Therefore,
the best position of the micro-fluid device for this system-6-