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	Vrije Universiteit Brussel, Ørsted (Denmark)
Citations	7
Analysis	Full AI Review Included

Technical Analysis and Documentation for Advanced MPCVD Diamond Optics

Executive Summary

This documentation analyzes the application of a tunable freeform-segmented reflector within a microfluidic system for high-sensitivity Surface-Enhanced Raman Scattering (SERS) and conventional Raman detection. The system focuses on achieving robust confocal behavior and superior background suppression.

Novelty: Integration of a numerically designed, segmented freeform reflector (30 mm diameter, NA=1.15) into a multi-layer microfluidic chip, enabling confocal Raman detection.
Material Limitations Identified: The current polymer (PMMA) chip and metal (Brass/NiP/Au) reflector introduce measurable background signal (PMMA) and rely on ultra-precision turning for moderately low roughness (RMS 14.7 nm).
Performance Metrics (Conventional): Achieved Noise-Equivalent-Concentration (NEC) of approximately 20 mM for aqueous urea and KNO₃ solutions, confirming moderate sensitivity.
Performance Metrics (SERS): Integration of a custom Au-coated nanopillar SERS substrate enabled high sensitivity, discriminating Rhodamine B (RhB) solutions down to 10 µM.
Confocality: The system demonstrated effective background suppression with a factor (SF) greater than 8, validated by the use of 100 µm core Multi-Mode Fibers (MMF).
6CCVD Advantage: High-purity MPCVD diamond (SCD/PCD) offers a superior, Raman-inert alternative to PMMA and metal substrates, capable of achieving far lower background noise and excellent thermal management required for high-power NIR excitation (785 nm).

Technical Specifications

Data extracted from the research paper regarding the operational parameters and achieved performance metrics.

Parameter	Value	Unit	Context
Excitation Wavelength	785	nm	Near-Infrared (NIR) Diode Laser
Max Excitation Power (Conv. Raman)	150	mW	Used for conventional Raman spectroscopy
Max Excitation Power (SERS)	70	mW	Reduced power to avoid damaging Au nanopillars
Reflector Material (Base)	Brass/NiP	N/A	Base material, finished by diamond turning
Reflector Coating Thickness	50	nm	Sputtered Au layer for high NIR reflectivity
Reflector Reflectivity (NIR)	>95	%	After final Au coating
Reflector Diameter / NA	30 / 1.15	mm / N/A	Segmented freeform design
Reflector Surface Roughness (RMS)	14.7 ± 1.4	nm	Gold coated surface roughness
Microfluidic Channel Width	600	µm	Channel within the PMMA chip layers
Noise-Equivalent-Concentration (Urea)	19	mM	Detection limit for conventional Raman (SNR=1)
SERS Detection Limit (Rhodamine B)	10	µM	Enhanced sensitivity via Au nanopillars
Maximum Suppression Factor (SF)	>8	N/A	Background reduction using 100 µm MMF
SERS Substrate Enhancement Factor	Up to 2.4 x 10⁶	N/A	Factor achieved by the Si nanopillar/Au structure

Key Methodologies

The study relies on precise optical design, non-sequential modeling, and advanced micro-fabrication techniques for both the optical component and the SERS platform.

Freeform Reflector Design: Calculated via a numerical approach based on Fermat’s principle to achieve precise segmentation and focus (center, middle concave, marginal segments).
Optical Modeling and Simulation: Non-sequential ray tracing using OpticStudio (Zemax) and the Henyey-Greenstein model to simulate scattering behavior, confocality (FWHM), background suppression, and alignment tolerance.
Segmented Reflector Fabrication: Pre-fabrication of brass base, followed by electroless Nickel Phosphorus (NiP) coating for hardness. Ultra-precision diamond turning was used to create the segmented surface profile, followed by 50 nm Au sputtering for >95% NIR reflectivity.
Microfluidic Chip Fabrication: Three layers of PMMA (1 mm thickness each) were laser cut to define the fluidic channel (600 µm width, 6 mm detection chamber), and then bonded using UV curing adhesive.
SERS Substrate Fabrication: An undoped single crystal silicon wafer was processed using maskless reactive ion etching (RIE) to form aperiodic nano-pillars (50-80 nm width, 600 nm height), followed by 200 nm Au coating via electron beam evaporation.

6CCVD Solutions & Capabilities

6CCVD identifies this research as a high-potential application area where the unique properties of chemical vapor deposition (CVD) diamond offer significant performance improvements over conventional materials (PMMA, brass, NiP).

Applicable Materials for Replication and Enhancement

To replicate this advanced Raman platform with drastically improved performance, 6CCVD recommends substituting the PMMA chip and metal reflector materials with high-purity MPCVD diamond:

Component Requirement	Current Material	Recommended 6CCVD Solution	Technical Advantage
Microfluidic Substrate/Chip	PMMA (High Raman Background)	Optical Grade SCD or PCD	Diamond is intrinsically Raman-inert, providing near-zero background signal and maximizing the Suppression Factor (SF).
High-Precision Reflector	Brass/NiP/Au (RMS 14.7 nm)	Optical Grade PCD or SCD Wafer	MPCVD diamond offers superior mechanical stability and polishability, achieving surface roughness of Ra < 1 nm (SCD) or Ra < 5 nm (PCD).
SERS Integration Platform	Silicon Wafer	Heavy Boron-Doped Diamond (BDD)	BDD combines high conductivity and electrochemical stability, ideal for novel SERS/electrochemical detection integration, replacing Si as a more robust substrate.

Customization Potential

The success of the analyzed paper hinges on the integration of high-precision optics (30 mm diameter) and thin-film metalization. 6CCVD’s in-house capabilities directly address these engineering requirements:

Large-Area Substrates: 6CCVD provides Polycrystalline Diamond (PCD) plates/wafers up to 125 mm in diameter, which are fully compatible with the required 30 mm reflector size and allow for multi-device integration on a single substrate.
Custom Optical Profiles: While the paper used diamond turning on NiP, diamond substrates can be post-processed via Laser Cutting and Polishing to achieve complex freeform segments and high-precision facets necessary for optimal NA (1.15) collection efficiency.
Integrated Metalization Services: The reflector requires a 50 nm Au coating for high NIR reflectivity. 6CCVD offers in-house deposition of noble metals including Au, Pt, Pd, Ti, W, and Cu, allowing for precise application and bonding layers directly onto the highly polished diamond surface.
Thickness Control: 6CCVD offers tight control over layer thickness, providing high-purity diamond films ranging from 0.1 µm up to 500 µm for critical optical applications.

Engineering Support

This research demonstrates the need for extremely low background and high thermal stability in complex Raman-on-chip systems, especially when scaling up excitation power or reducing integration time (e.g., SERS measurements used 70 mW power and 5s integration time).

6CCVD’s in-house PhD engineering team specializes in material science and photonics integration. We offer comprehensive consultation to transition similar advanced microfluidic projects from conventional materials (PMMA, brass) to MPCVD diamond, specifically focusing on maximizing the Suppression Factor (SF) by leveraging diamond’s exceptional Raman transparency and thermal conductivity.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

References

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