07184264.pdf

STu4K.7.pdf

CLEO:2015 © OSA 2015

On-chip Integrated Differential Optical Microring

Biosensing Platform Based on a

Dual Laminar Flow Scheme

Dongwan Kim

1,*

, Paula Popescu

, Mark Harfouche

, Jacob Sendowski

, Maria-Eleni Dimotsantou

, Richard

Flagan

, and Amnon Yariv

1,2

Department of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, USA

Department of Biomedical Engineering, University of Southern California, Los Angeles, California 90089, USA

Department

of Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA

dongwan.kim@caltech.edu

Abstract:

We propose an on-chip integrated di

fferential optical silicon nitride microring

biosensing platform which uses a dual laminar flow scheme. This platform reduces the fabrication

complexity involved in the fabrication of the reference resonator.

OCIS codes:

(280.4788) Optical sensing and sensors; (140.4780) Optical resonators

Introduction

Microfabricated optical resonator biosenso

rs, compatible with CMOS technologies and easily incorporated with

microfluidics, are good candidates for integration into portable electronic devices and commercial bench-top systems

[1, 2]. However, practical instruments for assay and molecu

lar binding measurements must be robust and external

environmental changes must be accounted for. Traditionally, differential measurements, utilizing a reference

resonator covered by either a SU-8 polymer or a silicon oxide (SiO

) cladding layer, have been used to correct for

temperature-induced signal drift and laser drift [3, 4]. In

this paper, we present an optical resonator biosensing

platform leveraging laminar flow conditions between the

two non-mixing solutions to realize a reliable and sensitive

differential measurement. In this platform, two resonators, on

e for sensing and the other fo

r reference, are exposed to

the aqueous environment. Then, two solutions, one containing the sample of interest and the other acting as a

reference, are flown in one common microfluidic channel,

with no disruption between the two fluid layers by

laminar flow conditions, and delivered to the sensing and

the reference resonator, respectively (Fig. 1(a)). This

platform reduces the fabrication complexity involved in fabricating the top cladding layer and opening of the sensing

window over the sensing resonator. We demonstrate the se

nsing capability of this platform by presenting the real-

time resonance peak shifts due to the refractive index ch

anges in a sodium chloride (NaCl) solution flown in bulk

over the sensor.

Fig. 1. (a) Schematic of the high-Q optical resonator biosensor using a dual laminar flow and the experimental setup. The refer

ence flow and

sensing flow in one common microfluidic channel are kept apart due to laminar flow conditions. (b) Schematic of the waveguide-r

esonator chip

drawn together with the microfluidic device structure. In blue is the flow channel network and in red are the control valves. T

he two left inlets are

used to deliver either the reference (deionized (DI) water, in this experiment) or sensing solution (NaCl) to the sensing reson

ator. One of the two

right inlets is used to deliver the reference solution (DI water) for the reference resonator. (c) Photograph of (left) the fab

ricated device where the

waveguide-integrated Si

microring resonator chip is bonded to the PDMS microfluidic device, (right) dime shown to provide scale. (d)

Photograph showing the two different dye solutions flowing onto three pairs of resonators with no disruption between the two fl

uid layers. Scale

bar, 1 mm.

Fabrications and experimental setups

Two optical microring resonators with 70 μm radius, 4 μm

width, and center to center spacing of 800 μm are

realized on a 250 nm thick silicon nitride (Si

) layer on top of 6 μm thick SiO

on a silicon handle. The 900 nm

wide waveguides and the microrings with the 400 nm coupling gap between them were patterned using electron

beam lithography on electron beam resist, ZEP520A. The patterns were transferred to the Si

layer using low DC-

STu4K.7.pdf

CLEO:2015 © OSA 2015

bias, inductively-coupled plasma reactive-ion etch (ICP-RIE) with SF

chemistry. For the liquid delivery, a

microfluidic polydimethylsiloxane (PDM

S) device is bonded on top of the Si

layer (Fig. 1(c)). The two-layer

microfluidic structure consists of a bottom flow layer and

a top control layer, and includes four inlets and one outlet,

each with a corresponding control valv

e (Fig. 1(b)). The reference (deionized

(DI) water, in this experiment) and

sensing solutions (NaCl) ar

e delivered to the sensing reso

nator using the two left inlets, while the reference solution

(DI water) is delivered to the reference resonator using one

of the two right inlets. The switching of the flow between

the different inlets is controlled by the microfluidic valves, which are actuated

by computer-controlled pressurized

solenoid valves [6]. To illustrate the laminar flow achieved on

this device, we introduce two different dye solutions

from two inlets into one common flow channel. Figure 1(d)

shows the established dual laminar flow for a successful

delivery of the different solutions to

the sensing and reference resonators. Th

e microring resonato

rs are characterized

using a 1064 nm optoelectronic fast linearly swept VCSEL

frequency laser with an optical frequency excursion of

300 GHz in 2 ms [5] that is coupled into the waveguide fr

om free space optics. The tr

ansparency of silicon nitride

and low water absorption at 1064 nm allows the microring

resonators to maintain high Q-factors in aqueous

environments, measured to be 1.4×10

and 2.0×10

(Fig. 2(a)). Using a Peltier th

ermoelectric cooler (TEC), the

temperature of the resonator chip

is fixed at 26 ºC (+/- 0.01 ºC).

Sensing methods and results

In order to demonstrate the actual sensing ability of this platform, sequentially diluted NaCl solutions of 2.5 mM, 5

mM, 10 mM, and 40 mM were flown over the resonators. Fi

rst, the baseline was established by the continuous flow

of the DI water from two inlets to both the sensing and reference resonator, followed by switching of the sensing

flow to the NaCl solution while maintaining the reference fl

ow of DI water. The sensing flow was then switched

back to DI water. The difference in the resonance fr

equency between the sensing resonator and the reference

resonator which was monitored in real-time is shown in Fig. 2(b). Concentrations of NaCl as low as 2.5 mM,

equivalent to 2.64×10

-5

refractive index units (RIU) with a sensit

ivity of -20.62 THz/RIU (77.81 nm/RIU), have

been measured (Fig. 2(c)). The on-chip valve switching an

d low dead volume in the two-layer microfluidic structure

result in a steady-state signal in about 20 seconds.

Fig. 2. (a) Transmission spectrum of the two high-Q microring resonators submerged in water measured with a 1064 nm linearly sw

ept VCSEL

(blue). The Lorentzian fits are shown in red. (b) Differential frequency shift versus time as NaCl sensing solution is flown ov

er the sensing

resonator while maintaining DI water over the reference resonator. (c) Resonance frequency shift versus concentrations of NaCl

solutions.

Conclusions

We demonstrate an on-chip integrated optical silicon nitride m

icroring resonator biosensing platform which uses a

dual laminar flow scheme removing the need for additional fabrication steps. The bulk sensing capability is

demonstrated using NaCl solutions of various concentrations

. Effort to enable specific binding sensing for biological

molecules using this platform is underway.

References

[1]

Muzammil Iqbal, et al., “Label

-Free Biosensor Arrays Based on Silicon Ring Resonators and High-

Speed Optical Scanning Instrumentation”,

IEEE J. Sel. Top. Quantum Electron.

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[2]

Samantha M. Grist, et

al., “Silicon photonic micro

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free biosensing”, Opt. Express,

(7), 7994-8006 (2013)

[3]

Kristinn B. Gylfason, et al., “On

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waveguide ring resonator refractive index sensor array”,

Opt. Express

(4), 3226-3237 (2010).

[4]

D.-

X.Xu, et al., “Real

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Naresh Satyan, et al., “Precise control of broadband frequency chirps using optoelectronic feedback”, Opt. Express

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(2009)

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Rafael Gómez-Sjöberg., Retrieved from https://sites.google.com/site/rafaelsmicrofluidicspage/valve-controllers/usb-based-contro

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