A novel and simplified procedure for patterning hydrophobic and hydrophilic SAMs for microfluidic devices by using UV photolithography

This work describes how selective patterning of hydrophobic and hydrophilic areas inside microchannels of microfluidic devices can be achieved by combining well-known chemical protocols and standard photolithography equipment (365 nm). Two techniques have been performed and compared. The first technique is based on the preparation of self-assembled monolayers of photocleavable organosilane and the second one on photoassisted grafting (365 nm) of self-assembled monolayers (SAMs) on a silicon or glass substrate. In the first case, we begin with monolayers carrying an o-nitrobenzyl function (hydrophobic area) that is photochemically cleaved, revealing a carboxylic acid group (hydrophilic area). The problem is that the energy necessary to cleave this monolayer is too high and the reaction time is more than 1 h with 50 mW/cm(2) irradiation flux. To overcome this practical disadvantage, we propose another approach that is based on the thiol-ene reaction with benzophenone as photoinitiator. In this approach, a monolayer of mercaptopropyltrimethoxysilane (MPTS) is prepared first. Subsequently, a hydrocarbon chain is photografted locally onto the thiol layer, forming a hydrophobic surface while the reminding unmodified thiol surface is oxidized into sulfonic acid (hydrophilic area). We demonstrated the feasibility of this approach and synthesized high-quality self-assembled monolayers by UV grafting with an irradiation time of 30 s at 365 nm (50 mW/cm(2)). The modified surfaces have been characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), AFM, and multiple internal reflection infrared spectroscopy (MIR-FTIR). The difference in the contact angles on the hydrophilic and hydrophobic surfaces reached a remarkable 77 degrees. We have also demonstrated that this method is compatible with selective surface grafting inside microfluidic channels.     

Microfluidic lithography of SAMs on gold to create dynamic surfaces for directed cell migration and contiguous cell cocultures

A straightforward, flexible, and inexpensive method to create patterned self-assembled monolayers (SAMs) on gold using microfluidics-microfluidic lithography-has been developed. Using a microfluidic cassette, alkanethiols were rapidly patterned on gold surfaces to generate monolayers and mixed monolayers. The patterning methodology is flexible and, by controlling the solvent conditions and thiol concentration, permeation of alkanethiols into the surrounding PDMS microfluidic cassette can be advantageously used to create different patterned feature sizes and to generate well-defined SAM surface gradients with a single microfluidic chip. To demonstrate the utility of microfluidic lithography, multiple cell experiments were conducted. By patterning cell adhesive regions in an inert background, a combination of selective masking of the surface and centrifugation achieved spatial and temporal control of patterned cells, enabling the design of both dynamic surfaces for directed cell migration and contiguous cocultures. Cellular division and motility resulted in directed, dynamic migration, while the centrifugation-aided seeding of a second cell line produced contiguous cocultures with multiple sites for heterogeneous cell-cell interactions.     

A simple approach to sensor discovery and fabrication on self-assembled monolayers on glass

Self-assembled monolayers (SAMs) on glass were used as a platform to sequentially deposit fluorophores and small molecules for ion sensing. The preorganization provided by the surface avoids the need for complex receptor design, allowing for a combinatorial approach to sensing systems based on small molecules. The resulting libraries are easily measured and show varied responses to a series of both cations and anions. This technology is transferable from the macro- to the microscale both via microcontact printing (microCP), where the fluorophore is printed onto a glass surface, and via direct attachment of the fluorophore to microchannel walls. The ease of miniaturization of this technology may make the generation of a wide variety of simple yet efficient microarrays possible.     

Multicolor, large-area fluorescence sensing through oligothiophene-self-assembled monolayers

We present a new strategy to realize self-assembled monolayers (SAMs) on quartz and silicon with a multicolour fluorescence pattern starting from a single, proton sensitive oligothiophene dye exposed at a defined pH. Fine tuning of the SAMs emission color over the entire visible range, including white, is demonstrated. Finally, integration of SAMs in patterned thin layer cells (TLCs) is exploited to demonstrate cation sensing potential in real devices.     

Combining microfluidic networks and peptide arrays for multi-enzyme assays

This paper reports the use of microfluidic networks (muFNs) to both prepare peptide microarrays and carry out label-free enzyme assays on self-assembled monolayers (SAMs) of alkanethiolates on gold. A poly(dimethylsiloxane) (PDMS) stamp fabricated with microchannels is used to immobilize a linear array of cysteine-terminated peptides onto SAMs presenting maleimide groups. The stamp is then reapplied to the SAM in a perpendicular direction to introduce enzyme solutions so that each solution can interact with an identical linear array of immobilized peptides. The muFNs enable multiple enzyme-substrate interactions to be simultaneously evaluated at a submicroliter scale, while the use of SAMs enables the use of MALDI mass spectrometry (MS) to analyze the enzyme activities. This paper demonstrates applications of this system for assaying multiple kinases and for profiling the activities of kinases and phosphatases in human K562 cell extracts. The combination of muFN, SAMs, and MS detection provides a flexible platform for assaying enzyme activities in biological samples.     

Photochemical properties and sensor applications of modified yellow fluorescent protein (YFP) covalently attached to the surfaces of etched optical fibers (EOFs)

Fluorescent proteins have the inherent ability to act as sensing components which function both in vitro and inside living cells. We describe here a novel study on a covalent site-specific bonding of fluorescent proteins to form self-assembled monolayers (SAMs) on the surface of etched optical fibers (EOFs). Deposition of fluorescent proteins on EOFs gives the opportunity to increase the interaction of guided light with deposited molecules relative to plane glass surfaces. The EOF modification is carried out by surface activation using 3-aminopropylthrimethoxysilane (APTMS) and bifunctional crosslinker sulfosuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (sulfo-SMCC) which exposes sulfhydryl-reactive maleimide groups followed by covalent site-specific coupling of modified yellow fluorescent protein (YFP). Steady-state and fluorescence lifetime measurements confirm the formation of SAM. The sensor applications of YPF SAMs on EOF are demonstrated by the gradual increase of emission intensity upon addition of Ca²⁺ ions in the concentration range from a few tens of micromolars up to a few tens of millimolars. The studies on the effect of pH, divalent cations, denaturing agents, and proteases reveal the stability of YFP on EOFs at normal physiological conditions. However, treatments with 0.5% SDS at pH 8.5 and protease trypsin are found to denaturate or cleave the YFP from fiber surfaces.     

巯基卟啉自组装膜的制备及应用研究进展

详细介绍了巯基卟啉自组装膜的几种制备方法,评述了不同制备方法的优缺点;对卟啉自组装膜在电荷转移、分子氧电催化、分子光电器件等领域的研究进展进行了评述,引用文献54篇。     

Patterned self-assembled beads in silicon channels.

A novel technique enabling selective bead trapping in microfluidic devices without the use of physical barriers is presented in this paper. It is a fast, convenient and simple method, involving microcontact printing and self-assembly, that can be applied to silicon, quartz or plastic substrates. In the first step, channels are etched in the substrate. The surface chemistry of the internal walls of the channels is then modified by microcontact printing. The chip is submerged in a bead slurry where beads self-assemble based on surface chemistry and immobilize on the internal walls of the channels. Silicon channels (100 microm wide and 50 microm deep) have been covered with monolayers of streptavidin-, amino- and hydroxy-functionalized microspheres and resulted in good surface coverage of beads on the channel walls. A high-resolution pattern of lines of self-assembled streptavidin beads, as narrow as 5 microm, has also been generated on the bottom of a 500 microm wide and 50 microm deep channel. Flow tests were performed in sealed channels with the different immobilized beads to confirm that the immobilized beads could withstand the forces generated by water flowing in the channels. The presented results indicate that single beads can be precisely positioned within microfluidic devices based on self-assembly which is useful as screening and analysis tools within the field of biochemistry and organic chemistry.     

在微流控芯片上合成对甲氧基苯甲醛肟

本文用负压进样的方法, 在自制的玻璃微流控芯片中进行了对甲氧基苯甲醛和盐酸羟胺合成对甲氧基苯甲醛肟的相转移反应. 测定了不同反应时间的产率, 并与常规方法进行了比较. 讨论了相接触面积和塞流对产率的影响.     

Characterization of self-assembled monolayers of porphyrins bearing multiple-thiol and photoelectric response

Self-assembled monolayers(SAMs) of thiol-derivatized porphyrin molecules on Au substrate have attracted extensively interest for use in sensing,optoelectronic devices and molecular electronics.In this paper,tetra-[p-(3-mercaptopropyloxy)-phenyl]porphyrin was synthesized and self-assembled with thiol on Au substrate for porphyrin SAMs(PPS 4).The electrochemical results demonstrated that PPS 4 could form excellent SAMs on gold surface.Self-assembled nanojunctions of PPS 4 were fabricated by using gold nanogap electrodes(gap width:ca.100 nm).With the light on/off,the nanojunctions showed current high/low as nanometer scaled photo switch.     

Screening of the effect of surface energy of microchannels on microfluidic emulsification

We report the results of a systematic study of the effect of the surface energy of the walls of microchannels on emulsification in parallel flow-focusing microfluidic devices. We investigated the formation of water-in-oil (W/O) and oil-in-water (O/W) emulsions and found that the stability of microfluidic emulsification depends critically on the preferential wetting of the walls of the microfluidic device by the continuous phase. The condition for stable operation of the device is, however, different than that of complete wetting of the walls by the continuous phase at equilibrium. We found that W/O emulsions form when the advancing contact angle of water on the channel wall exceeds theta approximately 92 degrees. This result is unexpected because at equilibrium even for theta < 92 degrees the microchannels would be completely wet by the organic phase. The criterion for the formation of W/O emulsions (theta > 92 degrees) is thus more stringent than the equilibrium conditions. Conversely, we observed the stable formation of O/W emulsions for theta < 92 degrees, that is, when the nonequilibrium transition to complete wetting by oil takes place. These results underlie the importance of pinning and the kinetic wetting effects in microfluidic emulsification. The results suggest that the use of parallel devices can facilitate fast screening of physicochemical conditions for emulsification.     

Fluorescent film sensor for copper ion based on an assembled monolayer of pyrene moieties

We previously reported the construction of a family of fluorescent film sensors for organic copper salts by covalently coupling polycyclic aromatic hydrocarbons on epoxy-terminated self-assembled monolayers on glass plate surfaces. Here we investigate the sensing properties and mechanism of covalently coupling pyrene on a glass plate surface via a long flexible "Y" type spacer. X-ray photoelectron spectroscopy (XPS) and fluorescence spectra measurements demonstrate the covalent attachment of pyrene in our adlayer. Compared with those results obtained in the previous studies, this new film sensor did not show highly selectivity for organic copper salts, which can be attributed to the introduction of sulfonyl groups connecting the pyrene moieties and the spacers. The presence of sulfonyl units made the microenvironments of pyrene relatively hydrophilic and thus showed less screening effect for inorganic ions. The specificity and reversibility of the film sensor toward Cu (II) made it attractive for sensing applications.     

Click chemistry by microcontact printing on self-assembled monolayers: a structure-reactivity study by fluorescence microscopy

Microcontact printing (μCP) has developed into a powerful tool to functionalize surfaces with patterned molecular monolayers. μCP can also be used to induce a chemical reaction between a molecular ink and a self-assembled monolayer (SAM) in the nanoscale confinement between stamp and substrate. In this paper, we investigate the Huisgen 1,3-dipolar cycloaddition, the Diels-Alder cycloaddition and the thiol-ene/yne reaction induced by μCP. A range of fluorescent alkyne inks were printed on azide SAMs and fluorescence microscopy was used to monitor the extent of the 1,3-dipolar cycloaddition on a glass substrate. The rate of cycloaddition depends on the reactivity of the alkyne and on the presence of Cu(I). The cycloaddition is accelerated by Cu(I) but it also proceeds readily in the absence of Cu(I). In addition, a range of fluorescent diene inks were printed on alkene SAMs on glass. In this case, fluorescence microscopy was used to monitor the rate of the Diels-Alder cycloaddition as well as its retro-reaction. Finally, fluorescent thiol inks were printed on alkene SAMs on glass, and fluorescent alkenes and alkynes were printed on thiol SAMs. It is shown that reactions by μCP follow structure-reactivity relationships similar to solution reactions. Under optimized conditions all reactions lead to dense microarrays of addition products within minutes of printing time.     

微流控芯片操纵传输及实时监测单细胞量子释放

微流控芯片技术用于细胞生化分析已引起了广泛关注.Harrison等首次在微流控芯片上对细胞群体进行操纵、传输及反应.yang等在微流控芯片上操纵细胞群体的排列,并用荧光检测细胞群体摄取钙的反应.至今还未见到微流控芯片对单个细胞进行操纵传输、定位及实时监测的报道.单细胞受激释放的监测对探索生物体神经传导具有重要意义.     

Multistep liquid-phase lithography for fast prototyping of microfluidic free-flow-electrophoresis chips

We present a fast and versatile method to produce functional micro free-flow electrophoresis chips. Microfluidic structures were generated between two glass slides applying multistep liquid-phase lithography, omitting troublesome bonding steps or cost-intensive master structures. Utilizing a novel spacer-less approach with the photodefinable polymer polyethyleneglycol dimethacrylate (PEG-DA), microfluidic devices with hydrophilic channels of only 25 μm in height were generated. The microfluidic chips feature ion-permeable segregation walls between the electrode channels and the separation bed and hydrophilic surfaces. The performance of the chip is demonstrated by free-flow electrophoretic separation of fluorescent xanthene dyes and fluorescently labeled amino acids.     

Strong resistance of oligo(phosphorylcholine) self-assembled monolayers to protein adsorption

In this work, we demonstrate the strong resistance of oligo(phosphorylcholine) (OPC) self-assembled monolayers (SAMs) to protein adsorption and cell adhesion. OPC SAMs were characterized using X-ray photoelectron spectroscopy (XPS), and protein adsorption was measured using a surface plasmon resonance (SPR) sensor. Results are compared with those of phosphorylcholine (PC) SAMs. Despite the existence of negative charge on OPC SAMs and the simple synthesis procedure of OPC thiols, OPC SAMs resist protein adsorption as effectively as or better than PC SAMs formed from highly purified PC thiols. The ease of their preparation and the effectiveness of their function make OPC SAMs an attractive alternative for creating nonfouling surfaces.     

Enantioselective analysis of melagatran via an LSPR biosensor integrated with a microfluidic chip

The impact of chiral compounds on pharmacological and biological processes is well known. With the increasing need for enantiomerically pure compounds, effective strategies for enantioseparation and chiral discrimination are in great demand. Herein we report a simple but efficient approach for the enantioselective determination of chiral compounds based on a localized surface plasmon resonance (LSPR) biosensor integrated with a microfluidic chip. A glass microfluidic chip with an effective volume of ～0.75 μL was fabricated for this application. Gold nanorods (AuNRs) with an aspect ratio of ～2.6 were self-assembled onto the surface of the inner wall of the chip to serve as LSPR transducers, which would translate the analyte binding events into quantitative concentration information. Human α-thrombin was immobilized onto the AuNR surface for enantioselective sensing of the enantiomers of melagatran. The proposed sensor was found to be highly selective for RS-melagatran, while the binding of its enantiomer, SR-melagatran, to the sensor was inactive. Under optimal conditions, the limit of detection of this sensor for RS-melagatran was found to be 0.9 nM, whereas the presence of 10?000-fold amounts of SR-melagatran did not interfere with the detection. To the best of our knowledge, this is the first demonstration of an LSPR-based enantioselective biosensor.     

Reversible covalent patterning of self-assembled monolayers on gold and silicon oxide surfaces

This paper describes the generation of reversible patterns of self-assembled monolayers (SAMs) on gold and silicon oxide surfaces via the formation of reversible covalent bonds. The reactions of (patterned) SAMs of 11-amino-1-undecanethiol (11-AUT) with propanal, pentanal, decanal, or terephthaldialdehyde result in dense imine monolayers. The regeneration of these imine monolayers to the 11-AUT monolayer is obtained by hydrolysis at pH 3. The (patterned) monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, contact angle and electrochemical measurements, and atomic force microscopy. Imines can also be formed by microcontact printing of amines on terephthaldialdehyde-terminated substrates. Lucifer Yellow ethylenediamine was employed as a fluorescent amine-containing marker to visualize the reversible covalent patterning on a terephthaldialdehyde-terminated glass surface by confocal microscopy. These experiments demonstrate that with reversible covalent chemistry it is possible to print and erase chemical patterns on surfaces repeatedly.     

A comparative analysis of localized and propagating surface plasmon resonance sensors: the binding of concanavalin a to a monosaccharide functionalized self-assembled monolayer

A comparative analysis of the properties of two optical biosensor platforms: (1) the propagating surface plasmon resonance (SPR) sensor based on a planar, thin film gold surface and (2) the localized surface plasmon resonance (LSPR) sensor based on surface confined Ag nanoparticles fabricated by nanosphere lithography (NSL) are presented. The binding of Concanavalin A (ConA) to mannose-functionalized self-assembled monolayers (SAMs) was chosen to highlight the similarities and differences between the responses of the real-time angle shift SPR and wavelength shift LSPR biosensors. During the association phase in the real-time binding studies, both SPR and LSPR sensors exhibited qualitatively similar signal vs time curves. However, in the dissociation phase, the SPR sensor showed an approximately 5 times greater loss of signal than the LSPR sensor. A comprehensive set of nonspecific binding studies demonstrated that this signal difference was not the consequence of greater nonspecific binding to the LSPR sensor but rather a systematic function of the Ag nanoparticle's nanoscale structure. Ag nanoparticles with larger aspect ratios showed larger dissociation phase responses than those with smaller aspect ratios. A theoretical analysis based on finite element electrodynamics demonstrates that this results from the characteristic decay length of the electromagnetic fields surrounding Ag nanoparticles being of comparable dimensions to the ConA molecules. Finally, an elementary (2 x 1) multiplexed version of an LSPR carbohydrate sensing chip to probe the simultaneous binding of ConA to mannose and galactose-functionalized SAMs has been demonstrated.     

Synthesis of fluorescent long-chain thiols/disulfides as building-blocks for self-assembled monolayers preparation

New symmetrical disulfides together with the corresponding thiols bearing fluorescent end-groups have been synthesized as building-blocks for self-assembled monolayers (SAMs). The synthesis has been accomplished starting from aromatic nitrogen heterocycles in three steps. The conversion of the tosylated intermediate into the final disulfide is accomplished by use of sodium hydrogen sulfide (NaSH). Both products (thiols and disulfides) were isolated and characterized.