Herein, we describe the fabrication and operation of a double-layer microfluidic system made of polydimethylsiloxane (PDMS). We demonstrate the potential of this device for trapping, directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures.
The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. Herein, we introduce a microfluidic-based technology, employing a double-layer microfluidic device, which can trap and localize in situ and ex situ synthesized structures on microfluidic channel surfaces. Crucially, we show how such a device can be used to conduct controlled chemical reactions onto on-chip trapped structures and we demonstrate how the synthetic pathway of a crystalline molecular material and its positioning inside a microfluidic channel can be precisely modified with this technology. This approach provides new opportunities for the controlled assembly of structures on surface and for their subsequent treatment.
Molekylære materialer har længe været undersøgt i det videnskabelige samfund på grund af deres brede antal ansøgninger på områder såsom molekylær elektronik, optik og sensorer 1-4. Blandt disse organiske ledere er en særligt spændende klasse af molekylære materialer på grund af deres centrale rolle i fleksible displays og integrerede funktionelle enheder 5,6. Imidlertid er metoder, der anvendes for at muliggøre elektronisk ladningstransport i molekylær-baserede materialer begrænset til dannelsen af charge transport komplekser (CTCs) og charge transport salte (CTSS) 7-10. Ofte CTCs og CTSS genereres ved elektrokemiske fremgangsmåder eller ved direkte kemiske redoxreaktioner; processer, der hæmmer en kontrolleret transformation af donor- eller acceptor-dele til mere komplekse arkitekturer hvor multifunktionalitet kan undfanget. Følgelig belysning af nye systematiske metoder til den styrbare generering og manipulation af molekylær-basend materialer fortsat en stor udfordring inden for materialevidenskab og molekylær teknik, og hvis det lykkes, vil uden tvivl føre til nye funktioner og nye teknologiske anvendelser.
I denne sammenhæng har mikrofluide teknologier for nylig blevet anvendt til at syntetisere molekylære-baserede materialer på grund af deres evne til at kontrollere varme- og masseoverførsel samt omsætningen-diffusion volumen af reagenser i en syntetisk proces 11,12. Forenklet sagt i kontinuerlige strømme og ved lave Reynolds tal en stabil grænseflade mellem to eller flere reagenser strømme kan opnås, hvilket giver dannelsen af en velkontrolleret reaktionszone inde i strømningsvejen, hvor blanding kun forekommer gennem diffusion 13-16. Faktisk har vi tidligere ansat laminare strømninger at lokalisere den syntetiske pathway af krystallinske molekylære materialer såsom koordinering polymerer (CPS) inde mikrofluidkanaler 17. Selv om denne metode har vist great løfte i at realisere nye CP nanostrukturer, den direkte integration af sådanne strukturer på overflader, samt kontrolleret kemisk behandling efter deres dannelse er endnu ikke realiseret på stedet 18. For at overvinde denne begrænsning, har vi for nyligt vist, at aktiveringen af mikrofluide pneumatiske bure (eller ventiler) inkorporeret i tolags mikrofluidenheder fordel kan anvendes i denne henseende. Da pionerarbejde Quake gruppe 19, har mikrofluide pneumatiske ventiler ofte blevet anvendt til enkelt-celle indfangning og isolation 20, enzymatiske aktivitet undersøgelser 21, indfangning af små væskevolumener 22, lokalisering af funktionelle materialer på overflader 23 og proteinkrystallisation 24. Vi har imidlertid vist, at dobbelt lag mikrofluidenheder kan anvendes til at fælde, lokalisere og integrere in situ dannede strukturer for at udlæse komponenter, og på overflader 18. Endvidere har vi også påvist, at en sådan teknologi kan anvendes til at udføre kontrollerede kemiske behandlinger på fanget strukturer, der gør det muligt både, "mikrofluid assisteret ligandudveksling" 18 og kontrolleret kemisk doping af organiske krystaller 18,25. I begge tilfælde kunne CTCs syntetiseres under kontrollerede mikrofluide betingelser, og i den seneste undersøgelse kunne multifunktionalitet blive beskrevet i det samme materiale stykke. Heri udviser vi under udførelsen af disse to lag mikrofluidenheder anvender dye-laden strømme, generere og styre koordineringen sti i en CP samt dens lokalisering på overfladen af en mikrofluid kanal og endelig vurdere kontrolleret kemiske behandlinger på on-chip indespærrede strukturer.
The reported approach can be easily modified to fabricate different valve shapes to afford other applications such as fluid confinement. Indeed, the flexibility of this protocol also allows for modification of the thickness of the bottom layer, and thereby of the PDMS membrane, from a couple of tens to a few hundreds of microns to fulfill any application of interest. Moreover, dimensions of structures in each layer of the device can be optimized for the desired application and various heights of structures on the master molds can be simply achieved by spinning the photoresist at different velocities. Spinning the photoresist at a higher speed results in thinner structures.
To better implement the protocol, a clean room environment for the fabrication of the master molds is substantially essential; otherwise, the fabrication procedure will lead to defective master molds and thereby to unusable microfluidic devices. Two critical aspects should be emphasized in this protocol: i) the constant temperature of the oven that needs to be adjusted to 80 °C and ii) the programmed time period between processes that has to be complied accurately. Any modification of temperature and time frame in the protocol might lead to non-bonded chips, and thus, to non-functional devices.
The “turbulent free” conditions typically encountered in microfluidic systems have recently been employed for the generation of microstructures or molecular materials inside30 and outside single layer microfluidic chips31. In double-layer microfluidic chips, the laminar flow regime, and hence, the interface generated between continuous co-flows can be manipulated using pneumatic cages18,28. These devices also provide for effective control over the synthetic pathway, which in turn leads to precise localization and trapping on surfaces18.
As mentioned earlier, pneumatic actuation in double-layer microfluidic chips has been previously employed for various applications such as cell trapping20, enzymatic activity studies21 and protein crystallization24. However, the main objective of the reported approach is to propose a platform to be used for trapping and directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures18,25.
The described method does not only allow trapping of anisotropic structures but can be used to localize particles onto surfaces. Future studies can be effectively directed towards the design of new valve shapes for additional application in biology, materials science and sensor technologies. The combination of different valve shapes as well as altered channel heights and membrane thicknesses can be employed to fulfill specific applications, such as chemical studies based on diffusional mixing and the localization of material growth.
A further application of the described microfluidic platforms is in the controlled chemical doping of crystals, which can lead to a rationalized formation of interfaces in crystalline structures19. This approach also provides for a wide range of post-treatments of on-chip trapped structures; a methodology that will undoubtedly open new horizons in materials engineering.
It is important to underline that the number of technologies enabling controlled chemical reactions under dynamic conditions and onto crystalline matter are very limited at present, hence making this approach very attractive in materials-related fields. However, a major limitation of this technology is the use of PDMS. PDMS elastomer is incompatible with many organic solvents, which limits the number of reactions that can be conducted inside these microfluidic chips. In future, the development of other elastomers that can tolerate and be stable against a broader number of organic solvents will be highly required in order to expand this field of research to other materials and chemistries.
The authors have nothing to disclose.
Authors would like to thank the financial support from Swiss National Science Foundation (SNF) through the project no. 200021_160174.
High resolution film masks | Microlitho, UK | – | Features down to 5um |
SU8 photoresist | MicroChem Corp., USA | SU8-3050 | – |
Silicon wafers | Silicon Materials Inc., Germany | 4" Silicon Wafers | Front surface: polished, Back surface: etched |
Silicone Elastomer KIT (PDMS) | Dow Corning, USA | Sylgard® 184 | – |
Spinner | Suiss MicroTech, Germany | Delta 80 spinner | – |
UV-Optometer | Gigahertz-Optik Inc., USA | X1-1 | – |
Mask Aligner | Suiss MicroTech, Germany | Karl Suss MA/BA6 | – |
SU8 developer | Micro resist technology GmbH, Germany | mr-Dev 600 | – |
Trimethylsilyl chloride | Sigma-Aldrich, Switzerland | 386529 | ≥97%, CAUTION: Handle it only under fume hood. |
Biopsy puncher | Miltex GmBH, Germany | 33-31AA-P/25 | 1 mm |
Biopsy puncher | Miltex GmBH, Germany | 33-31A-P/25 | 1.5 mm |
Glass coverslip | Menzel-Glaser, Germany | BB024040SC | 24 mm × 60 mm, #5 |
Laboratory Corona Treater | Electro-Technic Products, USA | BD-20ACV | – |
PTFE tubing | PKM SA, Switzerland | AWG-TFS-XXX | AWG 20TFS, roll of 100 m |
Silicone rubber tubing | Hi-Tek Products, UK | – | 1 mm I.D. |
neMESYS Syringe Pumps | Cetoni GmbH, Germany | Low Pressure (290N) | – |
High resolution camera | Zeiss, Germany | Axiocam MRc 5 | – |
Fluorescent inverted microscope | Zeiss, Germany | Axio Observer A1 | Operable at two wavelengths i.e. 350 nm and 488 nm |
Green polystyrene fluorescent particles | Fisher Scientific, Switzerland | 11523363 | Size: 5.0 um, solid content: 1% |
Silver nitrate (AgNO3) | Sigma-Aldrich, Switzerland | 209139 | ≥99.0%, |
L-Cysteine (Cys) | Sigma-Aldrich, Switzerland | W326305 | ≥97.0%, |
Disposable weighing dish | Sigma-Aldrich, Switzerland | Z154881 | L × W × H : 86 mm × 86 mm × 25 mm |
Disposable weighing dish | Sigma-Aldrich, Switzerland | Z708593 | Hexagonal, Size XL |
Plastic spatula | Semadeni, Switzerland | 3340 | L × W : 135 mm x 14 mm |
Dye, Bemacron ROT E-G | Bezema, Switzerland | BZ 911.231 | Red |
Stereomicroscope | Wild Heerbrugg, Switzerland | Wild M8 | 500x magnification |
Disposable scalpels | B. Braun, Switzerland | 233-5320 | Nr. 20 |
L-Ascorbic acid | Sigma-Aldrich, Switzerland | A4403 | – |