DSpace at VNU: Direct Measurement of Silver Anoparticles Suspended in Aqueous Solution by Liquid Electrode Plasma - Atomic Emission Spectrometry

23 Direct Measurement of Silver Nanoparticles Suspended in Aqueous Solution by Liquid Electrode Plasma - Atomic Emission Spectrometry Le Van Chieu1,*, Nguyen Hoang Tung2 1 VNU Projec

23

Direct Measurement of Silver Nanoparticles Suspended

in Aqueous Solution by Liquid Electrode Plasma - Atomic

Emission Spectrometry

Le Van Chieu1,*, Nguyen Hoang Tung2

1

VNU Project Management Department, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam

2

Institute of Environmental Technology, Vietnam Academy of Science and Technology (VAST),

18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam

Received 10 August 2017

Revised 17 August 2017; Accepted 22 September 2017

Abstract: This paper presents a quantitative measurement of silver nanoparticles in aqueous suspension

by liquid electrode plasma atomic emission spectrometry (LEP-AES) The dependence of the LEP-AES signal intensity on voltage-pulse height and duration was investigated The detection limit and

coefficient of variation (CV) were also measured The CV attained a minimum value of 7% for a pulse height of 1080 V and a pulse duration of 7 ms The detection limit (3σ) of silver nanoparticles by LEP-AES, under these optimal conditions, was calculated from a calibration curve to be 0.23 µg/g

Keywords: Silver nanoparticle, quantitative measurement, liquid electrode plasma, atomic emission spectrometry

There is an increasing demand for a compact

analysis system that would be capable of

measuring the concentration of elements in

solution; correspondingly, several miniaturized

plasma-based approaches have been reported

recently [1-5] Among these, liquid electrode

plasma atomic emission spectroscopy

(LEP-AES) has emerged as a simple and highly

sensitive analysis method for detecting elements

in aqueous solution [6-9] This handheld device

is easy to use The detection of metallic ions

such as Na+ and Li+ ions dissolved in nitric acid

_



Corresponding author Tel.: 84-904119229

Email: lechieu@vnu.edu.vn

https://doi.org/10.25073/2588-1094/vnuees.4123

has been demonstrated [2], and that of Cd2+ ions

to a precision of 0.3 ppm has been reported In addition, Pb and Cu were also investigated by LEP-AES with the detection limits of 4 and 0.52 µg/L, respectively [3] The principle of the LEP-AES is illustrated in figure 1

The quantitative analysis of the different elements can be performed by measuring the intensities of their characteristic emission peaks

On the other hand, detecting metallic elements in

a solid-state environment is also of interest for its potential for nanoparticle (NP) science applications Various techniques such as atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma atomic emission spectrometry (ICP-AES) can yield quantitative measurements of NPs

FI (Top view)

Power supply

Pt electrode

Narrow part

Depth is 50µm

(Detailed top view) (Side view)

600µm 30µm

(a) (b)

+

+

-M M

M

M

M

-Plasma

+

+

-+

M M

M

+ + + +

+ Ag Ag Ag

-+

+

-M

M M

-(c)

Figure 1 Mechanism of emission process The

sample suspension is introduced into the narrow part

of the chip device, in the absence of air bubbles The

platinum electrodes are placed at both ends of the

flow channel Voltage pulses are applied to the

platinum electrodes (a) The constriction region on the

chip is magnified in (b) The principle of the emission

by the silver nanoparticles is illustrated in (c)

However, LEP-AES is a promising approach

for achieving more compact and cost-effective

measurements Metallic ions have been

successfully detected by liquid electrode plasma;

however, the application to metallic NPs has so

far not been demonstrated In particular, the

essential process of atomization of the NPs into

individual metallic atoms has not been

investigated to date In this study, the detection

limit of silver NPs using LEP-AES was

systematically investigated

2 Experiment

2.1 Reagents

Silver NP standards, a kind of suspended

solution, with various diameters 20, 40, and 60

nm (BBI, United Kingdom), and silver-ion

standard solution (KANTO, Japan) were used for

studying detection of the silver NPs by

LEP-AES Phosphate-buffered saline (PBS) solution

(pH 7.4) for diluting the silver standards was prepared from pure salts of Na2HPO4, NaH2PO4, and NaCl (WAKO, Japan) Milli-Q water was used throughout all experiments in this study

2.2 Apparatus

Sedimentation of the silver NPs was performed by a ultracentrifuge equipment (TOMY, USA) A quartz microchip

(LepiCuve-C cuvette) (Micro Emission Ltd., Japan) was used for measuring silver NPs, and optical emissions of this NPs were recorded by a spectrometer (Andor Technology, UK, SR-3031-A) and a CCD camera (Andor Technology, UK, Newton)

2.3 Sample preparation

1.09 g of Na2HPO4, 0.368 g of NaH2PO4, and 9 g of NaCl were dissolved into 100 ml distilled water to create 0.1 M PBS solution (pH 7.4) A 3 µg/g silver NP suspension (diameter 20 nm), diluted from the initial standard with the PBS solution, was used for optimizing the conditions to detect the silver NPs The experiments, directly detecting silver NPs by LEP-AES, were carried out on the suspensions

of 5 µg/g of the silver NPs with diameters to be 0 (corresponding to silver ion solution), 20, 40, and 60 nm A calibration curve with 5 points (0, 0.5, 1, 2, and 3 µg/g) was diluted from a initial 20nm-silver NP standard with the PBS solution

2.4 Experimental setup for direct detection of

Ag NPs by LEP-AES

The experimental setup for detecting the silver NPs by LEP-AES was schematically shown in Fig 2b The sample solution was carefully spiked into the microchannel with a syringe A voltage was applied across the channel and controlled by the pulsed power source This source supplied pulses of predefined intensities and durations (Fig 2a) The resulting plasma excited the silver atoms to generate emissions, which were then captured by an optical fiber and recorded by a spectrometer

FI FI

Pulse power source

Optical fiber

Spectrometer Syringe pump

Waste Computer

(a)

a - pulse height

b - pulse duration

c - interval time between each pulse

a

Total applied pulses

b c

Figure 2 Experimental setup (a) Parameters of the

voltage pulses, applied at both ends of the flow

channel, and generated by the pulsed power source

(b) Sample suspension spiked into the microchannel

with a syringe The optical emission by the silver

nanoparticles is captured by the optical fiber and

recorded by the spectrometer and the computer

3 Results and discussion

3.1 Investigation of the optimal conditions for

detecting the silver NP by LEP-AES

Pulse height dependency

The 3 µg/g silver NP (20 nm) suspension

was used to investigate the pulse height

dependence Each measurement consisted of ten

equal pulses, and lasted 7 ms at 4 ms intervals,

with a height of 800, 900, 950, 1000, 1050,

1080, 1100, 1150, and 1200 V Figure 3 shows

the average value of seven repeated

measurements for each pulse height There was a

clear emission peak for silver appearing at a

wavelength of 338 nm The increase in silver

peak intensities with the applied pulse height is

plotted in Fig 3 For the weak pulses (800-1000

V), the emission intensity was very low, whereas

at the other extreme pulses (1050-1200 V), it

was relatively high Each pulse height creates a

temperature to excite emission of the silver NP,

therefore the increase of pulse height leads to a

temperature rise, resulting in an increase of the

emission intensity However, at 1080 V, the

relative coefficient of variation (CV) for the

silver optical emission intensity was minimum (Fig 4) The optimal pulse height was therefore taken at 1080 V

0 200 400 600 800

800 900 950 1000 1050 1080 1100 1150 1200

Pulse height [V]

Figure 3 Optical intensity of the silver nanoparticle emissions as a function of voltage pulse height

0 20 40 60 80

800 900 950 1000 1050 1080 1100 1150 1200

Pulse height [V]

Figure 4 Coefficient of variation of the silver optical emission intensity as a function of the pulse height

Pulse duration dependency

Using the same NP suspension as mentioned above, ten voltage pulses were applied for each measurement with a duration of 3, 4, 5, 6, 7, 8, 9, and 10 ms The time interval between each pulse was 4 ms, and the pulse height was set to 1080

V, and seven measurements were averaged Increasing pulse duration was expected to result

in an increase of the silver optical emssion intensity, however the result showed that the optical emission intensity of the silver NP depended non-monotonically on pulse duration (tp) Intensity of the silver NP was increased with increasing of the tp from 3 to 7 ms However, at the tp values above 7 ms, intensity

of the silver NP was decreased with incresing of the tp (Fig 5)

60

120

180

240

300

Pulse duration [ms]

Figure 5 Optical intensity of the silver nanoparticle

emissions as a function of the pulse duration

(The time interval between pulses is 4 ms)

0

20

40

60

80

100

Pulse application time [ms]

Figure 6 Coefficient of variation of the silver optical

emission intensity as a function of pulse duration.

The average CV, excluding the maximum

and the minimum values of the seven

measurements, was calculated for each tp value

(Fig 6), giving an average CV of 22.5% for

silver The CV values for the pulse duration of 3,

4, 5, 6, 7, 8, 9, and 10 ms were 93, 25, 23, 11, 7,

13, 18, and 57%, respectively For the tp value at

7 ms, the optical emission intensity of silver was

highest, and the CV was lowest The optimal

pulse duration was therefore taken at 7 ms

In conclusion, the pulse height and the pulse

duration of 1080 V and 7 ms were respectively

used for further experimental studies

3.2 Direct detection of the silver NPs by

LEP-AES

Two experiments including sedimentation of

the silver NPs from the solution and analysis of

size change of the silver NPs were performed to prove the direct detection ability of the silver NPs by LEP-AES

Deposition of the silver NPs from the solution

The silver NPs in the solution may maintain

an equilibrium between the nanoparticle type and the ion type Therefore, to eliminate the case

in which the silver spectrum received from LEP-AES was only emitted by the ion, the silver NPs were deposited from the solution And after the solution was separated into two parts, named the deposition and the emergent supernatant, for compairing the intensities of the silver optical emission Many reports indicated that the deposition of metallic NPs from solution was usually performed by a centrifugation method Two concentrations of the silver NPs (0.5 and 3 µg/g) with the size of 20 nm were centrifugated

at 14,000 rpm/min in 20 °C for 10 minutes The corresponding spectrum intensities were measured by LEP-AES as follows, ten pulses of the height of 1080 V and duration of 7 ms were applied at intervals of 4 ms The results showed that the intensities of the silver NPs in the depositional parts were significantly higher than those in the supernatants for the concentrations

of both 0.5 µg/g and 3 µg/g (Fig 7A and B) Therefore the silver NPs deposited by centrifugation were detected by LEP-AES

Analysis of size change of the silver NPs by LEP-AES

The suspensions of 5 µg/g of the silver NPs with diameters of 0 (solution of silver ion), 20,

40, and 60 nm were demonstrated by LEP-AES For measuring the corresponding spectrum intensities of the silver NPs, the pulse height of

1080 V and the duration of 7 ms were applied at the intervals of 4 ms The data in Fig 8 showed averages of three sets of pulses The emission intensity decreases as the NP diameter increases and the smallest size of the silver NP (diameter

of 20 nm) has the highest intensity among all other sizes (expected the silver NP with “zero-diameter”) (Fig 8)

14

28

42

56

70

STD Below Upper

(A)

0

100

200

300

400

500

332 334 336 338 340

(B)

Wavelength [nm]

Figure 7 Intensities of the silver NPs in both the

deposition and the supernatant parts after using

centrifugation method, (A) – the silver NP

concentration of 0.5 µg/g, (B) – the silver NP

concentration of 3 µg/g.

0

240

480

720

960

1200

0nm 20nm 40nm 60nm

Size of AgNPs [nm]

Figure 8 Optical intensity of the silver nanoparticles

as a function of diameter A diameter of 0 represents

a silver ion solution

The results can be explained by steric

inhibition of the NP sizes When the NP size

increases, density of the NP around the plasma

environment of LEP-AES may decrease For this

reason, the intensity of the silver NP with the

coarser size was lower than that with the finer

size The dependence of the silver intensity on

the NP size also is an evidence for directly detecting the silver NP by LEP-AES

Studies about both the deposition of the silver NPs from the solution and dependence of the silver intensity on the NP size confirmed the direct detection of the silver NPs by LEP-AES

Calibration curve and the detection limit

For the purpose of calibration, a 10 µg/g of the silver NP standard suspension with diameter

20 nm was diluted in the PBS to form suspension concentrations of 0, 0.5, 1, 2, and 3 µg/g The measuring conditions for each concentration by LEP-AES were performed as follows Ten pulses of the height of 1080 V and duration of 7

ms were applied at intervals of 4 ms To assess the reproducibility, measurements were repeated seven times for each concentration An increase

in the silver NP emission intensity with the concentration was observed (Fig 9)

The calibration curve is shown in Fig 10 with the correlation coefficient of 0.959 The

%CV was calculated as 29.2% for the silver NP concentrations by the silver NP calibration curve The limit of detection (LOD) for the silver

NP was estimated by using the equation LOD = 3σ/s, where σ is the standard deviation of the measurement data of blank solution and s is the slope of calibration curve On the basis of Fig

10, the silver NP detection limit was calculated

to be 0.23 µg/g

0 100 200 300 400

Wavelength [nm]

0 ppm 0.5 ppm

1 ppm

2 ppm

3 ppm

AgNPs 338nm

Figure 9 Optical emission spectra of the silver NPs

at each point in the calibration curve (the silver NPs peak measuring at the wavelength of 338 nm)

y = 73.67x + 23.08 R² = 0.959

-50

50

150

250

350

0 0.5 1 1.5 2 2.5 3 3.5

Conc of AgNPs [ppm]

Figure 10 Calibration curve of the silver

nanoparticle intensity.

4 Conclusions

The silver NP suspensions were directly

quantified by LEP-AES with the detection limit

of 0.23 µg/g The results also confirmed that the

sensitivity of the silver NP depends on both the

height and duration of the applied voltage pulses

Our results suggest that LEP-AES may be a

potential method for measuring other metallic

NPs Further investigations on tagging various

antibodies with the silver NP are needed for

using LEP-AES as a detection technique in

biological applications

References

[1] Iiduka A, Morita Y, Tamiya E and Takamura Y, MicroTAS 2004, Vol 1, The Royal Society of Chemistry, Cambridge, 2004, pp 423-425 [2] Banno M, Tamiya E and Takamura Y 2009 Anal Chim Acta 634 L153

[3] Matsumoto H, Iiduka A, Yamamoto T, Tamiya E and Takamura Y, Proceedings of MicroTAS 2005 Conference, Vol 1, Transducer Research Foundation, Boston, 2005, pp 427-429

[4] Kumagai I, Matsumoto H, Yamamoto T, Tamiya

E and Takamura Y, Proceedings of MicroTAS

2006 Conference, Vol 1, Tokyo, 2006, pp

497-499

[5] Kumai M, Nakayama K, Furusho Y, Yamamoto T and Takamura Y 2009 Bunseki Kagaku 58 L561 [in Japanese]

[6] Kagaya S, Nakada S, Inoue Y, Kamichatani W, Yanai H, Saito M, Yamamoto T, Takamura Y and Tohda K 2010 Anal Sci 26 L515

[7] Yamamoto T, Kurotani I, Yamashita A, Kawai J and Imai S 2010 Bunseki Kagaku 59 L1125 [in Japanese]

[8] Nakayama K, Yamamoto T, Hata N, Taguchi S and Takamura Y 2011 Bunseki Kagaku 60 L515 [in Japanese]

[9] Jo K W, Kim M G, Shin S M and Lee J H 2008 Appl Phys Lett 92 L1503

Đo trực tiếp hạt nano bạc trong dung dịch bằng phương pháp plasma điện cực lỏng kết nối phổ phát xạ nguyên tử

Lê Văn Chiều1, Nguyễn Hoàng Tùng2

1 Ban quản lý các dự án, Đại học Quốc gia Hà Nội, 144 Xuân Thủy, Hà Nội, Việt Nam

2

Viện Công nghệ môi trường, Viện Hàn lâm Khoa học và Công nghệ Việt Nam (VAST),

18 Hoàng Quốc Việt, Cầu Giấy, Hà Nội, Việt Nam

Tóm tắt: Bài báo này trình bày phép đo định lượng hạt nano bạc trong dung dịch bằng plasma điện

cực lỏng kết nối phổ phát xạ nguyên tử (LEP-AES) Nghiên cứu này đã khảo sát sự phụ thuộc cường độ tín hiệu LEP-AES vào chiều cao xung và thời gian áp xung Giới hạn phát hiện và hệ số biến thiên (CV) cũng được khảo sát CV của chiều cao xung tại 1080 V đạt giá trị thấp nhất là 7% tại thời gian áp xung 7

ms Giới hạn phát hiện (3σ) của hạt nano bạc bằng LEP-AES tại các điều kiện tối ưu đã được tính toán từ đường chuẩn là 0,23 µg/g

Từ khóa: Hạt nano bạc, đo định lượng, plasma điện cực lỏng, phổ phát xạ nguyên tử