A Tunable Freeform-Segmented Reflector in a Microfluidic System for Conventional and Surface-Enhanced Raman Spectroscopy

At a Glance

Metadata	Details
Publication Date	2020-02-25
Journal	Sensors
Authors	Qing Liu, Michael Stenbæk Schmidt, Hugo Thienpont, Heidi Ottevaere
Institutions	Technical University of Denmark, Ørsted (Denmark)
Citations	7
Analysis	Full AI Review Included

Executive Summary

Core Innovation: A miniaturized, confocal Raman/SERS system integrating a numerically designed freeform-segmented reflector with a PMMA microfluidic chip.
Background Suppression: The confocal design achieves high background suppression, demonstrating a Suppression Factor (SF) greater than 8 when using a 100 µm core Multi-Mode Fiber (MMF).
Conventional Raman Sensitivity: The setup achieved a Noise-Equivalent-Concentration (NEC) of approximately 19 mM for aqueous urea and 18 mM for potassium nitrate (KNO₃).
SERS Enhancement: Integration of an Au-coated silicon nanopillar SERS substrate (Enhancement Factor up to 2.4 x 10⁶) significantly boosted sensitivity.
Trace Detection: The SERS microfluidic chip successfully discriminated 0.1 mM (6 ppm) urea and 0.1 mM (10 ppm) KNO₃ solutions, demonstrating suitability for trace analysis.
Fabrication Method: The reflector was manufactured using ultra-precision diamond turning on a brass/NiP base, while the PMMA chip layers were fabricated via laser cutting, ensuring high precision and integration capability.

Technical Specifications

Parameter	Value	Unit	Context
Reflector Numerical Aperture (NA)	1.15	-	Segmented freeform design
Reflector Diameter	30	mm	Overall size
Reflector Tunability	4	mm	Axial adjustment range
Excitation Wavelength	785	nm	Diode laser source
Excitation Power (Conventional Raman)	150	mW	Power into microfluidic chip
Excitation Power (SERS)	70	mW	Reduced power to avoid substrate damage
Reflector Coating	50 nm Au on NiP	nm	Reflectivity > 95% for NIR light
Gold Surface RMS Roughness	14.7 ± 1.4	nm	Measured via non-contact profilometer
PMMA Chip Layer Thickness	1	mm	Top and bottom sealing layers
Fluidic Channel Width	600	µm	Middle layer channel dimension
Minimum Sample Volume	30	µL	Consumption per experiment
Conventional Raman NEC (Urea)	19	mM	Noise-Equivalent-Concentration
Conventional Raman NEC (KNO₃)	18	mM	Noise-Equivalent-Concentration
SERS Detection Limit (Urea/KNO₃)	0.1	mM	Achieved with SERS substrate
SERS Substrate Enhancement Factor	Up to 2.4 x 10⁶	-	Aperiodic Au nanopillars
Max Suppression Factor (SF)	> 8	-	Achieved using 100 µm MMF
Confocality FWHM (Z-axis)	155.6	µm	Simulated for 400 µm MMF
Raman Peak (Urea)	998	cm^-1	Main band in aqueous solution
Raman Peak (KNO₃)	1040	cm^-1	Main band in aqueous solution

Key Methodologies

Optical Design and Simulation:
- The freeform segmented reflector profile was calculated using a numerical approach based on Fermat’s principle.
- Non-sequential ray tracing simulations were performed in OpticStudio (Zemax) using the Henyey-Greenstein model to define scattering volumes and assess confocality and background suppression.
Reflector Fabrication:
- The brass base was pre-fabricated and coated with a Nickel Phosphorus (NiP) layer via electroless plating to increase hardness and corrosion resistance.
- The segmented freeform surface was generated using ultra-precision diamond turning.
- A final 50 nm thick Gold (Au) layer was applied via sputtering coating to ensure >95% reflectivity for Near-Infrared (NIR) light.
Microfluidic Chip Fabrication:
- Three 1 mm thick PMMA layers were fabricated using laser cutting.
- The layers were bonded together using UV curing adhesive to form the fluidic channel (600 µm width, 6 mm diameter detection chamber).
SERS Substrate Fabrication:
- Undoped single crystal silicon wafers were processed using maskless Reactive Ion Etching (RIE) to form aperiodic nanopillars (50-80 nm width, ~600 nm height).
- A 200 nm thick Au layer was deposited onto the nanopillars using electron beam evaporation.
- The resulting substrates were laser cut into 4 mm x 4 mm pieces and integrated into the microfluidic chamber.
Experimental Setup and Measurement:
- A 785 nm diode laser was used for excitation, coupled with external optics (lenses, band-pass filter, dichroic mirror, beam expander).
- Raman scattering was collected by the segmented reflector and coupled into a Multi-Mode Fiber (MMF) acting as a confocal pinhole (100, 200, or 400 µm core diameters tested).
- Spectra were acquired using a spectrometer equipped with a TE cooled CCD detector and an 830 L/mm blaze grating.

Commercial Applications

Biomedical and Clinical Diagnostics: High-sensitivity detection of metabolites (e.g., urea) and biomarkers in low-volume biological samples, crucial for kidney function monitoring and general health screening.
Point-of-Care (POC) Testing: The compact, robust nature of the optofluidic chip and external optics makes the system suitable for miniaturized, portable Raman instruments for rapid, decentralized analysis.
Environmental and Water Quality Monitoring: Trace analysis of inorganic salts (like nitrates/nitrites) and organic pollutants at the parts-per-million (ppm) level, leveraging the SERS enhancement capability.
Pharmaceutical and Chemical Process Monitoring: Quantitative analysis of liquid solutions and chemical reaction kinetics in microfluidic reactors, benefiting from the high background suppression and confocal detection.
Micro-Optics and Photonics Integration: The freeform segmented reflector design provides a blueprint for integrating complex, high-NA optics directly into lab-on-chip platforms, reducing reliance on bulky external lenses.

View Original Abstract

We present a freeform-segmented reflector-based microfluidic system for conventional Raman and Surface-Enhanced Raman Scattering (SERS) analysis. The segmented reflector is directly designed by a numerical approach. The polymer-based Raman system strongly suppresses the undesirable background because it enables confocal detection of Raman scattering through the combination of a freeform reflector and a microfluidic chip. We perform systematic simulations using non-sequential ray tracing with the Henyey-Greenstein model to assess the Raman scattering behavior of the substance under test. We fabricate the freeform reflector and the microfluidic chip by means of ultra-precision diamond turning and laser cutting respectively. We demonstrate the confocal behavior by measuring the Raman spectrum of ethanol. Besides, we calibrate the setup by performing Raman measurements on urea and potassium nitrate solutions with different concentrations. The detection limit of our microfluidic system is approximately 20 mM according to the experiment. Finally, we implement a SERS microfluidic chip and discriminate 100 µM urea and potassium nitrate solutions.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s20051250

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