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Biomicrofluidics / Volume 6 / Issue 1 / REGULAR ARTICLES
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Biomicrofluidics 6, 014105 (2012); http://dx.doi.org/10.1063/1.3677369 (9 pages)

A nanoporous optofluidic microsystem for highly sensitive and repeatable surface enhanced Raman spectroscopy detection

Soroush H. Yazdi and Ian M. White

Fischell Department of Bioengineering, University of Maryland, College Park, Maryland 20742, USA

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(Received 21 November 2011; accepted 24 December 2011; published online 13 January 2012)

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages a nanoporous microfluidic matrix to improve the SERS detection performance by more than two orders of magnitude as compared to a typical open microfluidic channel. Although it is a growing trend to integrate optical biosensors into microfluidic channels, this basic combination has been detrimental to the sensing performance when applied to SERS. Recently, however, synergistic combinations between microfluidic functions and photonics (i.e., optofluidics) have been implemented that improve the detection performance of SERS. Conceptually, the simplest optofluidic SERS techniques reported to date utilize a single nanofluidic channel to trap nanoparticle-analyte conjugates as a method of preconcentration before detection. In this work, we leverage this paradigm while improving upon the simplicity by forming a 3D nanofluidic network with packed nanoporous silica microspheres in a microfluidic channel; this creates a concentration matrix that traps silver nanoclusters and adsorbed analytes into the SERS detection volume. With this approach, we are able to achieve a detection limit of 400 attomoles of Rhodamine 6G after only 2 min of sample loading with high chip-to-chip repeatability. Due to the high number of fluidic paths in the nanoporous channel, this approach is less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. In addition, fabrication of this microsystem is quite simple, as nanoscale fabrication is not necessary. Finally, integrated multimode fiber optic cables eliminate the need for optical alignment, and thus the device is relevant for portable and automated applications in the field, including point-of-sample and point-of-care detection. To illustrate a relevant field-based application, we demonstrate the detection of 12 ppb of the organophosphate malathion in water using the nanofluidic SERS microsystem.

© 2012 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. EXPERIMENTAL
    1. Materials
    2. Preparation of silver colloid
    3. Fabrication of the optofluidic device
    4. Scanning electron microscopy
    5. SERS measurements
  3. RESULTS AND DISCUSSION
  4. CONCLUSION

RELATED DATABASES

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KEYWORDS, PACS, and IPC

Keywords

bioMEMS, bio-optics, dyes, fibre optic sensors, microchannel flow, nanofluidics, nanomedicine, nanoporous materials, optical cables, patient diagnosis, silver, surface enhanced Raman scattering

PACS

International Patent Classification (IPC)

  • A61B5/00

    Measuring for diagnostic purposes; Identification of persons

  • B81B

    Micro-structural devices or systems, e.g. micro-mechanical devices

  • B82B1/00

    Nano-structures

  • C09B

    Organic dyes or closely-related compounds for producing dyes; Mordants; Lakes

  • F15D

    Fluid dynamics, i.e. methods or means for influencing the flow of gases or liquids

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.3677369

PUBLICATION DATA

ISSN

1932-1058 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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Figures (click on thumbnails to view enlargements)

FIG.1
Nanofluidic trapping vs. open-channel microfluidics. (a) In an open channel, nanoparticles are poorly concentrated. (b) A nanochannel traps silver nanoparticles (Ag NPs) into the detection volume. (c) Packed silica spheres form a nanofluidic matrix, which is capable of trapping a high number of Ag NPs into a relatively large detection volume without clogging. An optical fiber can be aligned to the nanoporous matrix.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
(a) Micrograph showing packed microspheres and integrated fiber optic cables. (b) Silver nanoparticles (AgNPs) are trapped in the silica microsphere matrix. (c) Excitation and collection is performed by integrated fiber optic cables. (d) Experimental setup: the sample is loaded with a syringe pump. The fiber optic cables are connected to a diode laser and a Raman spectrometer.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
SEM micrograph of the silica microspheres packed into the microfluidic channel after running silver nanoclusters through the channel. The bright spots on the silica spheres are silver nanoclusters that became trapped in the matrix.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.4
Time-dependent accumulation of SERS signal as silver nanoparticles with adsorbed R6G are trapped and concentrated within the nanofluidic matrix. R6G concentration in colloid = 100 nM. Spectra are shifted vertically for visual clarity.

FIG.4 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.5
Within the nanoporous matrix, the SERS signal is greater than 250 times more intense as compared to the open channel. R6G concentration in colloid = 100 nM.

FIG.5 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.6
(a) Mean intensity of the 1509 cm−1 Raman peak for various R6G concentrations. Error bars represent standard deviation, N = 3. (b) Measured SERS signal after 4 μl of 100 pM R6G (400 attomoles) is loaded into the microchannel.

FIG.6 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.7
Recorded SERS spectra for greater than 145 ppm malathion in water (the solubility limit) in an open microfluidic channel and for 12 ppb malathion in water in the 3D nanofluidic channel. Arrows indicate the Raman peaks for malathion. The background signal is also shown to enable the clear identification of the malathion Raman peaks.

FIG.7 Download High Resolution Image (.zip file) | Export Figure to PowerPoint



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