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.
Molekylära material har länge studerats i det vetenskapliga samfundet på grund av deras breda antalet ansökningar inom områden som molekylär elektronik, optik och sensorer 1-4. Bland dessa organiska ledare är en särskilt spännande klass av molekylära material på grund av deras centrala roll i flexibla displayer och integrerade funktionella enheter 5,6. Men metoder som används för att möjliggöra elektronisk laddningstransport i molekylära baserade material begränsad till bildandet av komplex laddningstransport (CTCs) och laddningstransportalter (CTSS) 7-10. Ofta CTCs och CTSS genereras av elektrokemiska metoder eller genom direkta kemiska redoxreaktioner; processer som hindrar en kontrollerad omvandling av givar eller acceptorenheter till mer komplexa arkitekturer där multifunktionalitet kan tänkas. Följaktligen klargörandet av nya systematiska metoder för den styrbara generering och manipulation av molekyl-based material är fortfarande en betydande utmaning inom materialvetenskap och molekylteknik, och om det lyckas kommer utan tvekan att leda till nya funktioner och nya tekniska tillämpningar.
I detta sammanhang har mikroflödesteknik nyligen använts för att syntetisera molekylbaserade material på grund av deras förmåga att kontrollera värme- och massöverföring samt reaktions-diffusion volym av reagens under en syntesprocess 11,12. Enkelt uttryckt, i kontinuerliga flöden och vid låga Reynoldstal en stabil gränsyta mellan två eller flera reagensströmmar kan uppnås, vilket ger bildning av en väl kontrollerad reaktionszonen inuti flödesvägen, där blandning sker endast genom diffusion 13-16. I själva verket har vi tidigare anställd laminära flöden för att lokalisera den syntetiska vägen för kristallina molekylära material, såsom samordnings polymerer (CPS) inuti mikroflödessystem kanaler 17. Även om denna metod har visat great löfte att förverkliga nya CP nanostrukturer, direkt integration av sådana strukturer på ytor, liksom kontrollerad kemisk behandling efter deras bildning har ännu inte förverkligas på plats 18. För att övervinna denna begränsning, har vi nyligen visat att aktiveringen av mikroflödes pneumatiska burar (eller ventiler) som ingår i två skikt mikrofluidikanordningar med fördel kan användas i detta avseende. Sedan pionjärarbetet av Quakes grupp 19, har mikroflödes pneumatiska ventiler ofta använts för encelliga infångning och isolering 20, enzymatiska aktivitetsundersökningar 21, infångning av små vätskevolymer 22, lokalisering av funktionella material på ytor 23 och proteinkristallisering 24. Emellertid har vi visat att dubbelskikt mikrofluidikanordningar kan användas för att fälla, lokalisera och integrera in situ bildade strukturer för att läsa ut komponenter och på ytor 18. Vidare har vi också visat att en sådan teknik kan användas för att utföra kontrollerade kemiska behandlingar på fångade strukturer, som gör det möjligt både, "mikroflödesassisterad ligandutbyte" 18 och kontrollerad kemisk dopning av organiska kristaller 18,25. I båda fallen skulle CTC syntetiseras under kontrollerade mikroflödesförhållanden, och i den senaste undersökningen, kan multifunktionalitet beskrivas i samma materialstycke. Häri, visar vi resultatet för dessa dubbla lager mikrofluidikanordningar som använder färgämnesladdade flöden, generera och styra samordningsvägen för en CP samt dess lokalisering på ytan av ett mikrofluidkanalen och slutligen bedöma kontrollerade kemiska behandlingar på on-chip fångade 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 | – |