A PACKAGED OPTICAL SLOT-WAVEGUIDE RING RESONATOR SENSOR ARRAY
FOR MULTIPLEX ASSAYS IN LABS-ON-CHIP
K.B. Gylfason1, C.F. Carlborg1, A. Kaźmierczak2, F. Dortu2, H. Sohlström1, L. Vivien3, G. Ronan4, C.A. Barrios5,
W. van der Wijngaart1, and G. Stemme1
1KTH – Royal Institute of Technology, SWEDEN,
2Multitel a.s.b.l., BELGIUM,
3Université Paris-Sud 11, FRANCE,
4Farfield Group Limited, UK, and
5Universidad Politécnica de Madrid, SPAIN
ABSTRACT
We present the design, fabrication, and characterization of a packaged array of individually addressable slot-waveguide ring resonator sensors in a compact car- tridge for sensitive, label-free, multiplex assays. The novel use of a dual surface- energy adhesive film enables simple generic packaging method for multiple sensors in a single cartridge. The use of optical slot-waveguides, and drift compensation by on-chip light splitting to reference sensors, gives the best refractive-index limit of detection reported for planar ring resonator sensors.
KEYWORDS: Label-Free, Multiplex, Slot Waveguide, Ring Resonator, Packaging INTRODUCTION
Measurements of optical absorption and refractive index are established methods for label-free analysis in chemistry and biology. For reliable analysis, positive and negative controls, as well as drift compensation, is essential. Accordingly, there is a need for integrating multiple optical sensors in labs-on-chip. An abundance of sin- gle sensors has been shown [1], but few integrated multi-sensor solutions.
Planar waveguide ring resonators are easily integrated with on-chip optical and fluidic functions, have a small footprint, and can be fabricated by standard tech- niques, thus enabling mass production at low cost. Single ring resonators have dem- onstrated a detection limit of 1.8x10−5 refractive index units (RIU) [2]. Recently, five ring sensors were integrated on a single chip [3], but no microfluidic network was integrated and optical signal splitting was handled off-chip.
The introduction of a nano-metric slot along the length of planar waveguides in- creases light/sample interaction in ring resonators [4]. We previously reported the application of individual such slot-waveguide ring resonators, without integrated flu- idics, to refractive index sensing [5] and label-free biosensing [6].
EXPERIMENTAL
We assembled a compact cartridge (Figure 1 (b)) from an optical chip, a micro- fluidic layer and a hard plastic shell (PMMA) (Figure 1 (a)). The integrated optical functions (Figure 2) are micro-fabricated in silicon nitride on silicon. Each of the
six sensors (M1-M6) is individually addressable by a fluidic network in PDMS (Syl- gard 184) patterned by soft lithography. For assembly, the PDMS is bonded to the PMMA shell by a machined dual surface-energy adhesive film (Nitto Denko 5302A).
(a) (b)
Figure 1. (a) The cartridge consists of an optical chip in silicon, a fluidic layer in PDMS, and an adhesive film bonding the PDMS to the hard plastic shell. The cartridge is aligned to pins on a temperature-controlled aluminium platform. (b) A
cartridge photograph. Steel tubes provide fluid connection to the microchannel network. The laser access hole is visible on the top surface close to the long edge.
The optical chip surface is then activated in 3:1 H2SO4:H2O2 and bonded to the PDMS. Since the on-chip optical, fluidic, and thermal interfaces are designed for alignment tolerance, the cartridge is easily replaced and self-aligned in the read-out system, which consists of a single tunable laser (1310 nm wavelength, 1 pm resolu- tion), a linear photodiode array, and a temperature controlled platform (Figure 1 (a)).
Figure 2. Layout of the optical network. The six sensors are labeled M1-M6.
REF1 and REF2 are used for alignment. Inset are: (A) one of the six slot waveguide ring-resonators, with an enlargement of the ring coupling; (B) the multimode
inteference splitter; and (C) the input grating coupler.
RESULTS AND DISCUSSION
To demonstrate real-time multiplex operation, ethanol and methanol dilutions were simultaneously measured in channels M1 and M2, while M4 monitored drift of the DI
water baseline (Figure 3 (a)). The slope of the resonance shifts (Figure 3 (b)) reveals a refractive index sensitivity of 246 nm/RIU—the highest reported for a silicon nitride ring-resonator. Based on system noise (1.2 pm) and temperature drift, we calculate a detection limit of 7x10−6 RIU, which is, to our knowledge, the best value reported for a planar ring resonator sensor.
(a) (b)
Figure 3. (a) Resonance wavelength shift of sensors M1, M2, drift compensated by M4, during ethanol and methanol injections of decreasing mass concentration
into a running buffer of DI water in channel M1 and M2, respectively (b) Reso- nance wavelength shift as a function of refractive index of the injections in channels
M1 and M2. Both sensors exhibit a refractive index sensitivity of 246 nm/RIU.
CONCLUSIONS
We have demonstrated a ring-resonator sensor array integrated with a microflu- idic network. We experimentally demonstrate the best detection limit of planar ring- resonator sensors, thus enabling sensitive multiplex assays in labs-on-chip.
ACKNOWLEDGEMENTS
K.B. Gylfason acknowledges support of the Steinmaur Foundation, Liechten- stein. C.A. Barrios acknowledges support from the Spanish Ministry of “Educación y Ciencia” under Program “Ramón y Cajal”. This work is done within the FP6-IST- SABIO project (026554), funded by the European Commission.
REFERENCES
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[3] A. Ramachandran et al., "A universal biosensing platform based on optical mi- cro-ring resonators", Biosensors and Bioelectronics, 23, 939 (2008).
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