Patent Application
20150246336
The invention relates to the generation of molecular microarrays, such as DNA microarrays or peptide microarrays.
Microarrays have become established as practical high-throughput experimental tools in multiple fields of biology. Most microarrays are DNA microarrays, but microarrays for other biological macromolecules, such as RNA, proteins and carbohydrates, exist. One of the most successful and practical synthesis methods for the fabrication of microarrays at both commercial and laboratory scale is in situ, light-directed synthesis. The basic technology is inspired by the photolithographic process that is used to make silicon microchips. The monomer building blocks of the biological macromolecules to be included on the microarray are synthesized with a light-sensitive group, which drops off when illuminated, leaving a reactive site to which the next monomer can couple. By combining coupling chemistry and light exposure, very complex microarrays can be synthesized. Microarrays are currently available with up to 2.1 million different DNA sequences per array. The same technology has been applied to the synthesis of proteins, RNA, and carbohydrate microarrays, but DNA microarrays represent the commercially dominant form and are widely used.
There are currently four dominant forms of light-directed synthesis. (1) The original invention by Affymetrix relies on the use of the MeNPOC light-sensitive group on the 5-hydroxyl group of DNA phosphoramidites. The microarrays are synthesized on glass substrates, which are covered with metal masks, similar to photolithographic masks. Holes in the masks allow light to reach predetermined areas of the glass substrate, where the chemical reactions occur leading to DNA synthesis. A method for in situ synthesis of probes of a microarray using a mask set is disclosed in U.S. Pat. No. 7,844,940 B2. (2) In a refined method the metal masks were replaced by an optical system incorporating an array of micromirrors. In addition to changing the method to deliver light, the chemistry was improved by using the NPPOC light-sensitive group, which photochemical and photophysical characteristics enable more efficient synthesis. The micromirrors can be tilted to direct light to the synthesis surface. Maskless optical micromirror-based microarray synthesis is described in U.S. Pat. No. 8,030,477 B2 and in WO 99/63385 A1.
Further improvements have been made to improve synthesis quality. US 2007/0037274 A1 provides an improved optical system to generate microarrays comprising a flow cell with optimized materials and fluids with similar refractive indices to reduce undesired reflections.
US 2006/229824 A1 relates to sets of probes with differences in single nucleotides. Also disclosed is the manufacture of a microarray gel, which is sliced to provide copies of the micro-array.
WO 92/10092 A1 discloses a standard microarray faction method using a mask.
U.S. Pat. No. 7 956 011 B2 describes the parallel synthesis of a multiplicity of microarrays.
WO 99/42813 A1 relates to a patterning device using a micro-mirror system.
A review on microarray synthesis apparatuses, materials and methods is provided in Agbavwe et al., Journal of Nanobiotechnology, 9:57, 2011.
Although microarray synthesis has been optimized since the first conception, it remains an expensive process, which limits the potential practical applications of microarray technology It is therefore a goal of the present invention to provide a convenient and economical process for synthesizing microarrays to reduce costs per created microarray.
This goal has been solved by the present invention, which provides a method and means, in particular a flow cell, that allows the parallel synthesis of more than one microarray simultaneously with little modifications in the synthesis apparatus.
The invention provides a method of step-wise synthesizing copies of polymers of potentially different units on at least two solid carrier surfaces simultaneously in a preselected pattern, comprising
In a related aspect, the invention provides a flow cell for synthesizing copies of polymers of potentially different units on at least two solid carrier surfaces simultaneously in a pre-selected pattern, comprising
Further provided is a kit comprising the flow cell and a container comprising polymer units, preferably modified by a photosensitive moiety.
The present invention is further defined as in the claims. Preferred embodiments of the present invention are further described herein in the following and relate to the inventive method and flow cell and kit equally, wherein the flow cell or kit can be suitable for performing said method steps, e.g. being adapted for the method steps or by comprising means to perform said method steps. The flow cell or kit can be used in the course of the inventive methods. Each of the preferred features or embodiments can be combined with each other in especially preferred embodiments except in cases of exclusive alternatives.
The term “microarray” is used in the art as either an arrangement or as a solid substrate. Both expressions apply to the present invention, which provides a solid surface with such an arrangement. Usually herein “microarray” refers to the solid surface with a pattern of multiple immobilized polymer molecules thereon. The carrier surface is also referred to a substrate herein.
The innovation is to modify the flow cell to synthesize multiple microarrays, especially two microarrays, simultaneously. Surprisingly is has been found that focal depth of focused light is sufficient to illuminate more than one parallel surface and it is still possible to provide suitable fluidic systems that can provide fluid flow onto the active surfaces in a gap or channel with a width sufficiently within an adequate focal depth range. This modification is possible because very little light is absorbed and lost in the synthesis process. Several layers of support carriers (e.g. 2, 3, 4 or more) can be stacked and illuminated by the same light beam. Stacking can be performed in any order.
A further requisite to light illumination is contacting with the fluids comprising the synthesizing units that shall be attached to the surfaces. In principle it is possible to contact each layer sequentially or in parallel, with the same fluid or different fluids. Since only a small quantity of the units in a fluid are adsorbed to the surfaces in the binding reaction, the fluid can pass various surfaces sequentially, e.g. through several flow channels. Of course, the larger the total channel volume, the greater the amount of required chemicals will be. Therefore is also a goal to minimize the gap or channel distance. When using thinner gaps or channels, problems with the fluidics system may arise. Usually the fluid inlet for the fluids, e.g. a tube port, is wider in diameter than the gap distance. If the fluid inlet is within the gap, e.g. in the seal or gasket of the gap, flow turbulences may occur due to changing fluid flow rates dependent on the narrowing to the gap dimension (in comparison to the wider tube). Therefore in a preferred embodiment an opening for a fluid inlet and/or outlet of the fluid for contacting the first and second surface is within one of the carriers itself. E.g. the first and/or second carrier may contain holes with openings for the fluid inlet and/or outlet. The holes in the support can be in any size and can be made to fit the tubes. The inlet and/or outlet may have a diameter of 250 μm to 6 mm, preferably of 500 μm to 4 mm, of 800 μm to 3 mm or of 1 mm to 2 mm.
It is preferred to use a single channel in order to minimize reagent loss and requirement. Thus, in a preferred embodiment of the invention, in step iii) the fluid is passed through a single channel or gap between the layers of the carrier surfaces, wherein said first and second surfaces face said channel or gap on opposing sides. It is e.g. possible to use the top and the bottom wall of the fluid channel as surfaces for synthesis. If a second synthesis surface is as the backside of the synthesis flow cell, the same array is synthesized on both substrates, the only difference between the two arrays is that they are mirror images of each other. The mirror image layout of the second array does not limit its use in any way. After hybridization (or other applications of the microarray) and scanning the microarrays with a microarray scanner, the images can be rotated and flipped in an image editor (e.g. Photoshop) to have the same orientation. Because the two arrays are synthesized simultaneously with the same chemical reagents and light exposure, they are as similar as two microarrays can be, and therefore may have additional utility as matched pairs for experiments that would benefit from very close data comparisons. Thus, in preferred embodiments of the invention the pattern is displayed on the first surface as a mirror image of the pattern as displayed on the second surface. Since the synthesis time, equipment and reagents are shared between the two arrays, the synthesis cost and time of the second array are greatly reduced. Thus, in a further aspect of the invention a set of two microarrays is provided, with each microarray comprising immobilized polymers in a preselected pattern, wherein the patterns of the two microarrays are mirrorimages to each other.
The gap or channel in the flow cell is preferably secured by a seal or gasket. Thus, in preferred embodiments of the flow cell, it further comprises a seal or gasket between said first and second layer of carriers, which creates a gap between the carriers. The primary additional cost is the second microarray substrate and a single-use gasket creating the seal between the two microarrays. In preferred embodiments, the fluid inlets and/or outlets are not in the seal or gasket, but in a solid carrier in an area enclosed by the seal.
Because the microarrays are directed optically, the two surfaces where the microarrays are synthesized need to be separated by a gap within or not greatly exceeding the focal depth of the focal plane of the optical system. In addition, the gap between the two surfaces needs to be small in order to minimize the consumption of the solvents and reagents used in the synthesis. Both of these requirements can be met simultaneously by using a gap of limited dimensions. In preferred embodiments the layer distance between the first and second surface layer is at most 2 mm or at most 1.5 mm, preferably at most 1 mm, especially preferred at most 800 μm or at most 500 μm, even more preferred at most 250 μm, most preferred at most 150 μm or at most 100 μm. The layer distance between the first and second surface layer can be at least 20 μm, at least 30 μm, at least 40 μm or at least 50 μm. A preferred range is 30 μm to 150 μm, especially preferred 50 μm to 100 μm, e.g. about 70-80 μm. The precise optimum gap size is determined by the specific characteristics of the system. A smaller gap is preferred due to a lowering of the consumption of the solvents and reagents, as well as decreasing the requirements of depth-of-focus of the optical system. Smaller gaps are more difficult to engineer due to the thinness of the required gasket and the higher pressures necessary to pump solvents and reagents through the narrower gap.
The first carrier is transparent in the meaning that the light used in the inventive method substantially passes through said carrier material to cause photoreactions with the photoreactive moieties or compounds in the vicinity of the first and second carrier surface. A transparent carrier means one that allows passage of light in sufficient amounts in order to cause photoreactions on the second surface or in the fluid to cause a chemical reaction depending on light incidence on the second surface. Transparency can mean that at least 30%, preferably at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% of incident light to pass through. Preferably, also the second carrier is transparent. In addition, it is possible to place the carrier arrangement onto a block, which preferably is also transparent. Transparent materials help to minimize heating by light absorption. In especially preferred embodiments the space between such a block and the closest carrier, e.g. the second carrier, contains an anti-reflective material, e.g. a fluid with a similar refractory index, also referred to as “index-matching fluid”, as the transparent material, to minimize light scattering. Such a system is e.g. disclosed in US 2007/0037274 A1 (incorporated herein by reference). Alternatively or in addition, the space between the block and the second carrier (or other closest carrier) is filled with a light absorbing material, suitable to absorb the light used for activation, e.g. UV absorbers, such as beta carotene, 9-methylanthracene, riboflavin or combinations thereof. Thus in preferred embodiments of the invention an anti-reflective and/or light absorbing material, preferably a fluid, is provided behind the second carrier, with “behind” being a reference to the direction of light (photons) used during activation. The light absorber and anti-reflective material help to minimize back-scattering and allow the reduction of unwanted deprotection or activation in spots that are not to be irradiated. A suitable transparent material for the first and/or second carrier is glass, plastic or any other suitable microarray substrate. Preferably the block is of metal or glass, especially preferably the block is chemically resistant and transparent in the near ultra-violet and visible wavelengths, such as glass, quartz, fused silica or sapphire. Further materials for the carriers and/or block are disclosed in U.S. Pat. No. 6,329,143, U.S. Pat. No. 6,310,189; U.S. Pat. No. 6,309,831; U.S. Pat. No. 6,197,506; and U.S. Pat. No. 5,744,305, all of which are hereby incorporated by reference. For instance, the carriers may be a polymerized Langmuir Blodgett film, functionalized glass, Si, Ge, GaAs, GaP, SiO2, SiN4, modified silicon, or any one of a wide variety of gels or polymers such as (poly)tetrafluoroethylene, (poly)vinylidenedifluoride, polystyrene, polycarbonate, or combinations thereof. Other carrier materials will be readily apparent to those of skill in the art upon review of this disclosure. In a preferred embodiment the substrate is flat glass or single-crystal silicon. The carrier surface is preferably flat, especially at the level of 1 μm, especially preferred of 100 nm, 10 nm or 1 nm.
Without an index-matching and absorbing fluid, the best possibility to reduce reflected light is to use an antireflective coating on the back side of the transparent block. These coatings usually reduce the reflection from about 4% to about 1% of the incident light. This one percent light reflected back is currently the largest source of error in light-directed synthesis of microarrays. Preferably an absorbing fluid is provided between the block and the closest carrier, e.g. the second carrier. An absorbing fluid can reduce the reflected light to approximately zero.
Preferably the light pattern used for activating the surfaces is generated by an array of selectively tiltable micromirrors reflecting light in the preselected pattern dependent on the tilt of each mirror. A micromirror system is disclosed in Agbavwe et al (supra) and in U.S. Pat. No. 8,030,477 B2 and in WO 99/63385 A1 (all incorporated herein by reference) and can be used according to the invention. Lenses or curved mirrors can be positioned between the light source and the micromirror, which can be a micromirror array, or between the micromirror and the substrate.
It is further possible to generate said pattern by a mask comprising holes according to the preselected pattern. A system using light masks is disclosed in U.S. Pat. No. 7,844,940 B2 (incorporated herein by reference) which can be used according to the invention. A mask can be a solid mask, which a can be used in an array of masks to create different patterns or a alternatively, the mask can have fields than can be switched between transparent and blocking modes, such as an array of liquid crystals as are commonly used in LCDs.
It is further possible to generate said pattern with a laser as disclosed in US 2005/0079601 A1 (incorporated herein by reference). The laser can be used to sequentially illuminate spots of the pattern. Alternatively it is possible to use an array of lasers, wherein by activating and inactivating certain lasers creates the pattern. Furthermore it is possible to use an array of light emitting diodes similarly to an array of lasers.
The pattern is preferably a 2D pattern. Spots can be arranged in rows and columns in a preselected way, wherein each spot has bound a potentially different polymer as is synthesized according to the present invention.
The pattern may comprise or consist of spots. A single or each spot displayed onto the first and/or second surface may have a size of 5 μm to 3 mm, preferably of 10 μm to 2 mm, 15 μm to 1 mm, 25 μm to 800 μm, 40 μm to 600 μm, 50 μm to 400 μm or 60 μm to 200 μm.
The projected light can be light of multiple wave-lengths, e.g. light of a lamp, or monochromatic light, e.g. of a laser or light emitting diode. The light can be visible light or ultraviolet light, i.e. in the wavelength ranges of 200 nm to 400 nm or 400 nm to 800 nm or a combination thereof, i.e. of 200 nm to 800 nm. Further preferred wavelength ranges are from 200 nm to 300 nm, 300 nm to 400 nm, 400 nm to 500 nm, 500 nm to 600 nm, 600 nm to 700 nm or 700 nm to 800 nm or combinations thereof, especially preferred is the combination of 300 nm to 500 nm. Suitable lamps or other light sources can be selected by a skilled artisan, depending on the reactivity of the photosensitive moiety used. An example lamp is a Hg lamp. In preferred embodiments, the light is within or encompasses the wavelength range of 350 nm to 450 nm. Preferably the light has a bandwidth of 50 nm to 600 nm, preferably of 80 nm to 400 nm or 100 nm to 200 nm within the above mentioned wavelength ranges. It is also possible to use light with narrower bandwidths, such as lasers or light emitting diodes. E.g. the bandwidth can be from 1 nm to 50 nm, preferably 5 nm to 40 nm or 10 nm to 30 nm.
Preferably the light can be focused. To this end a light guide, e.g. a mirror or set of mirrors, or lenses can be used. Preferably the inventive method comprises focusing the light with a focus point between said first and second carrier surface. Preferably the focal point is in the middle third of the distance between the first and second surface. The light guide of the flow cell is preferably suitable for such focusing. Alternatively, spatially coherent light such as a laser light can be used.
The flow cell may e.g. comprise a light guide for projecting light though said first solid carrier onto the second solid carrier, preferably focusing the light with a focus point between said first and second carrier surface.
The light can catalyze a chemical reaction on the surfaces, e.g. an amino acid addition reaction or the addition, removal or crosslinking of organic or inorganic molecules or compounds, small or large. For example, during the addition of a nucleic or an amino acid residue, the light can deprotect the units comprising photosensitive moieties as protecting groups, e.g. phosphoramidite containing compounds. Therefore, in a preferred embodiment, the units as used in the inventive method or as provided in the kit can comprise photosensitive moieties preventing a binding reaction of further units to said units until exposed to said projected light. According to the embodiment step ii) can also be defined as projecting light in a preselected pattern onto the first carrier surface, thereby activating photosensitive moieties on the first surface, wherein the light passes through the transparent first solid carrier, and further projects onto the second carrier surface, thereby activating photosensitive moieties on the second surface. However the photosensitive moieties are not required to be bound to the surface. They can be provided in a fluid, not bound to the unit or surface. Unbound photosensitive moieties are in the following referred to as photosensitive compound. Alternatively, it is also possible to use or provide units comprising or chemically-labile groups preventing a binding reaction of further units until removed by an activated photosensitive compound. Suitable pairs of labile moieties and photosensitive compounds, which do not need to be bound to the units or the surface, exist in the art. An example is an acid sensitive group on the units as labile moiety and a photoacid as photosensitive compound, which reacts with a photon and creates an acid microenvironment upon radiation in the vicinity of the radiation, and in turn near a spot on a microarray that comprises the bound labile unit, which in turn becomes reactive to bind a further unit.
Suitable polymer units may comprise a nucleoside for synthesis of nucleic acid polymers or amino acids for synthesis of polypeptides. Polymer units may be naturally-occurring molecules or synthetic derivatives thereof. The units as used herein may refer to monomer units or preconnected conjugated monomers, e.g. dimers, trimmers, etc. Possible units are nucleosides, nucleotides or conjugated nucleic acids, including 1-mer, 2-mer, 3-mer, 4-mer, 5-mer, 6-mer or longer preprepared chains. Units can also be single amino acids, or dipeptides, tripeptides, etc. that are bond to the surface in this state. Nucleotide units can be selected from A, G, T, C, or U, and derivatives thereof, such as methylated C. Nucleotides are preferably DNA, RNA or LNA. Amino acids can be selected from Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, Val or any combination thereof, and derivatives thereof such as selenocysteine. Methods for protein microarray synthesis are e.g. detailed in WO 2012/126788 A1 (incorporated herein by reference).
The incentive method steps of sequentially ligating or binding units to the surface is preferably repeated until a polymer of the desired length and sequence, optionally on each spot, is created. A polymer can be composed of two or more covalently bonded monomers or units, and its molecular weight may be approximately 1,000 or less. The polymer may include approximately 2 through 500 monomers or units. More specifically, the polymer may include approximately 5 through 30 monomers or units.
Light can also be used for the crosslinking or mono-, bi-, or multi-functional binding groups or compounds to attach molecules such as fluorochromes, antibodies, carbohydrates, lectins, lipids, and the like, to the substrate surface or to molecules previously or concurrently attached to the substrate.
Preferred photosensitive moieties that undergo a chemical reaction upon light exposure are phosphoramidites, such as NPPOC, as used in the art. The preferred light dependent reaction is a deprotection of a chemical moiety on the unit that can then be conjugated with a further unit.
The last step in microarray synthesis may be an overall deprotection to remove all remaining protection steps without further adding additional units.
The first step in microarray synthesis may be attaching a first unit onto the carrier surface that contains reactive chemical groups for linkage with the units. Said chemical groups may be protected with a photosensitive moiety or chemically-labile moiety, which can be removed by action of light on a photosensitive compound, which in turn removes the chemically-labile moiety once activated by light, such as photoacids which act on acid sensitive moieties. Chemical groups may be any form of organic linker molecules such as C1-C30 linkers or any one of the above monomers such as nucleic acids or proteins, in particular poly(X)-chains wherein X is a nucleotide selected from A, T, C, G, U, preferably T. The chain may have any length, preferably 1 to 20 nucleotides or other monomers in length.
The invention further relates to a kit comprising the flow cell and a container comprising polymer units. Said polymer units can be any one as defined above, e.g. nucleotides or amino acids. The units are preferably modified by a photosensitive moiety or by chemically-labile moiety. The photosensitive moiety or the chemically-labile moiety can be removed by light to expose reactive sites on the units for further polymer synthesis as described above. In the kit, the units can be provided in dry form or in a fluid, e.g. an aqueous fluid. The kit may comprise different containers for different units, such as nucleotides selected from A, G, T, C, or U, or any amino acid, e.g. selected from Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, Val or any combination thereof, or derivatives thereof as described above. The kit preferably comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more different units, preferably from the ones listed above.
Microarrays according to the invention can be used as is known in the art. They can be used in nucleotide binding assays, such expression assays, e.g. to determine miRNA or mRNA in a sample, or in peptide recognition assays, e.g. the determine antibodies in a sample. Especially preferred are differential assays. In differential or comparative assays two microarrays are contacted with different samples, e.g. one sample of interest and one control sample. The inventive microarrays are particularly beneficial for use in differential assays especially when using a pair of microarrays that have been generated simultaneously as described herein. Simultaneously generated microarrays are near identical (except for being optionally mirror images), with minimal variance. The use of such a simultaneously generated set of microarrays minimizes background signals and improves comparative assays in sensitivity in detecting differences between the two samples. Preferably the inventive kit or flow cell comprises a set of at least two microarrays, wherein at least two microarrays are mirror images to each other and/or have been generated simultaneously according to the inventive method. Related thereto, the invention also provides the final product of the inventive method in a kit, in particular a kit comprising at least two microarrays with immobilized polymers in a preselected pattern, wherein the patterns of at least two microarrays are mirror image of each other and/or wherein the at least two microarrays have been produced simultaneously in the inventive method. This kit with at least two microarrays can be used as describe, especially for differential or comparative assays.
The inventive microarrays may comprise at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more different polymers. The polymers may be arranged in a 2D pattern comprising, e.g. comprising rows and columns, e.g. a pattern of at least 2×2, at least 3×3 or at least 4×4 spots. Especially in 2D patterns, the mirror-image pattern of one of the at least two microarrays is non-identical with the pattern of the other of the at least two microarrays. In 1D patterns, the mirror-image pattern of one of the at least two microarrays may be identical or non-identical with the pattern of the other of the at least two microarrays. Preferred is a at least a pair of non-identical mirror-imaged microarrays.
The present invention is further described in the figures and following examples, without being limited to these embodiments of the invention.
A flow cell for microarray synthesis has been manufactured as shown in
The fluid inlets and outlets into the chamber between the first and second substrate have a diameter of 1 mm. These inlets and outlets provided as holed drilled into the second substrate. These holes are sufficiently separated from the microarrays so as not interfere with microarray synthesis or use. In preliminary experiments it has been shown that it is not practical to introduce inlet and outlet flows through the gasket material since inlet and outlet openings of sufficiently small diameter to enter the chamber through the narrow gap distance between the microarray substrate cannot transport a sufficient volume of reagents to effectively fill or drain the chamber, nor to provide the necessary homogenous flow of solvents and reagents across the substrate surfaces.
For nucleotide synthesis on the substrates reference is made to
The reaction cell needs to position the two microarray substrates at the focal plane of the optical system. There is some tolerance to this positioning: the depth of focus of the imaging optics. The imaging optics are a 1:1 Offner relay system, an off-axis conjugate system composed of two spherical concentric mirrors, primary and secondary. The system was designed with a numerical aperture (NA) of 0.08 to result in a resolving power of 2.7 μm. This resolving power is sufficient since it is significantly smaller than the size of individual mirrors of the digital micromirror device (DMD), 13×13 μm, separated by a 0.7 μm gap, and is similar or better than those of most available microarray scanners. A low value of numerical aperture lowers the cost of the primary mirror, but more importantly, reduces the amount of scattered light originating from dust and imperfections in the optical system, which is proportional to NA2. Unintended photodeprotection, from scattering, diffraction and local flare, is the largest source of sequence error in light-directed microarray synthesis (Agbavwe et al. 2011, supra). The depth of focus is intrinsically limited by diffraction to <˜λ/NA2, ˜60 μm, but in practice, the positioning of the microarray substrates in the focal plane is somewhat less restricted due to limited resolution of microarray scanners. Therefore, the primary optical constraint in the simultaneous light-directed synthesis of microarray pairs is that the two substrates must be within ˜60-100 μm of each other, depending on the scanner resolution.
A secondary constraint is imposed by reagent delivery. A larger reaction cell volume requires larger flow rates of solvents and reagents, the consumption of which scales with cell volume. Since our original reaction cell (for synthesizing microarrays on a single surface) had a depth of 70 μm and worked well with a standard oligonucleotide synthesizer (Expedite 8909), we took this value as a starting point. Thus, the reaction cell should consist of two standard microarray substrates (75×25×1 mm) separated by a uniform gap of ˜70 μm. The microarray substrates form the entrance and exit windows for the ultraviolet light used in the synthesis. Reagents need to be introduced into this gap and to uniformly flow across the surface before exiting. The reaction cell assembly consists of a black anodized aluminum support block, a quartz block, the two microarray substrates, two gaskets, and a clamping frame and screws to hold the parts together. Reagent delivery tubes attach to the underside of the quartz block and connect to the oligonucleotide synthesizer.
The support block forms the rigid structure for the assembly of the reaction cell and allows for the reaction cell to be precisely positioned in the focal plane. Three alignment points make contact with ball-tipped, high-precision adjustment screws (Newport AJS127-0.5H) in the optical system. After initial adjustment of the screws, the reaction cell assembly can be quickly and reproducibly positioned. The support blocks hold a quartz block. The quartz block has four 0.8 mm through-holes (two inlets, two outlets) that are countersunk on the back side to accommodate microfluidics ports. The microfluidics ports (IDEX 6-32 Coned NanoPort Assemblies) were turned on a lathe to reduce their diameter to 6.4 mm, and attached within each countersunk hole with common cyanoacrylate adhesive. The front and back surfaces of the quartz block were machined to a surface parallelism error of <30 arc sec and polished to an optical flatness of λ/4 (Mindrum Precision). During reaction cell assembly, the lower gasket is placed on the quartz surface. This gasket forms the lower chamber, which can be filled via two of the fluidics ports. Prior to microarray synthesis, this chamber can be filled with an index-matching and light absorbing fluid to prevent light reflections from light exiting the reaction chamber. In the legacy reaction cell design, an antireflective coating on the back surface of the quartz block can reduce the back reflection to a minimum of about 0.25% when new, but this value is typically larger, ˜1%, due to the presence of dust, chemical films and scratches. This 0.25 to 1% value is sufficient to make this unintended light exposure the largest source of error after diffraction, but unlike diffraction, the error is not confined primarily to the gaps between microarray features (Agbavwe et al. 2011, supra). An alternative strategy to reduce back reflections is to fill the lower chamber with an index-matching fluid with dissolved chromophores which absorb the light exiting the reaction chamber, and which either convert the light to heat or Stokes shift it beyond the absorption band of the light-labile group.
The lower gasket has two holes that align with two of holes in the quartz block. These holes couple the corresponding fluidics ports to the microarray synthesis cell. This gasket is made from 250 μm thick Chemraz 584 perfluoroelastomer (FFKM), cut to shape with a laser cutter (Spirit GX). The microarray synthesis cell is a chamber consisting of two glass substrates separated by a very thin gasket. This chamber is accessed via two 1 mm holes, in the lower substrates, which align with the holes in the lower gasket.
The thickness of the upper gasket determines the depth of the photochemical reaction cell and therefore needs to be ˜70 μm thick, chemically resistant and sufficiently elastic to form a seal for the duration of the synthesis, up to ˜12 hours for an array of 70 mers. These requirements are quite exceptional and we were unable to find any references to such thin gaskets in the scientific or engineering literature. A perfluoroelastomer, such as Chemraz, would likely work, but the manufacturer is unable to make them thinner than 250 μm. We tried expanded polytetrafluoroethylene (PTFE), which is commonly used in gasket applications due to its chemical resistance and ability to compress to form a seal, but found seepage through the gasket, presumably due to it porous nature. In the end we found that the common PTFE tape used for plumbing applications works well. This tape is made from unsintered PTFE and is therefore sufficiently compressible to form a seal, but not porous. PTFE tape is made in many thicknesses and densities, which allowed for some experimentation. We initially used ˜100 μm (120 μm uncompressed) PTFE with a density of ˜1.4 g/cm3 (Gasoila yellow tape)—sintered PTFE has a density of about 2 g/cm3—but found some loss of focus when microarrays were scanned at a resolution of 2.5 μm. A wider focal range would be required for thicker gaskets. Switching to thinner and lower density PTFE tape (Gasoila Industrial Strength SD, ˜0.7 g/cm3) gave a thickness of ˜50 μm under compression. With this thickness, both of the paired arrays produce sharp scans with resolution limited only by the 2.5 μm pixel size of the scanner, and both reagent and helium flow sweep uniformly across the entire surface of both substrates. The 50 μm PTFE gaskets are also formed with a laser cutter. Because of their thinness, they are too delicate to be reusable, but can be made quickly and inexpensively.
Schott Nexterion Glass D slides functionalized with N-(3-triethoxysilylpropyl)-4-hydroxybutryamide (Gelest SIT8189.5). The arrays with holes were drilled with a 0.9 mm diamond bit and washed and rinsed in an ultrasonic bath prior to functionalization. The slides were loaded in a metal staining rack and completely covered with a 500 ml of a solution of 10 g of the silane in 95:5 (v/v) ethanol:water and 1 ml acetic acid. The slides were gently agitated for 4 hours and then rinsed twice for 20 min with gentle agitation in the same solution, but without the silane. The slides were then drained and cured overnight in a preheated vacuum oven (120° C.). After cooling to room temperature, the slides were stored in a desiccator cabinet until use. Microarrays were synthesized directly on the slides using a maskless array synthesizer, which consists of an optical imaging system that used a digital micromirror device to deliver patterned ultraviolet light near 365 nm to the synthesis surface. Microarray layout and oligonucleotide sequences are determined by selective removal of the NPPOC photocleavable 5′-OH protecting group. Reagent delivery and light exposures are synchronized and controlled by a computer. The chemistry is similar to that used in conventional solid-phase oligonucleotide synthesis. The primary modification is the use of NPPOC phosphoramidites. Upon absorption of a photon near 365 nm, and in the presence of a weak organic base, e.g. 1% (m/v) imidazole in DMSO, the NPPOC group comes off, leaving a 5′-terminal hydroxyl which is able to react with an activated phosphoramidite in the next cycle. The DNA sequences on the microarrays in this project were synthesized with a light exposure dose of 4.5 J/cm2, with coupling time of 40 s at monomer concentrations of 30 mM. After synthesis, the microarrays were deprotected in 1:1 (v/v) ethylenediamine in ethanol for two hours at room temperature, washed twice with distilled water, dried with argon, and stored in a desiccator until hybridization.
Microarrays were hybridized in adhesive chamber (SecureSeal SA200, Grace Bio-labs) with a solution consisting of 0.3 pmols 5′-CyS-labeled probe, 40 pg herring sperm DNA and 200 pg acetylated BSA in 400 μL MES buffer (100 mM MES, 1 M NaCl, 20 mM EDTA, 0.01% Tween-20). After 2 hrs of rotation at 42° C., the chamber was removed and the microarrays were vigorously washed in a 50 ml centrifuge tube with 30 ml non-stringent wash buffer (SSPE; 0.9 M NaCl, 0.06 M phosphate, 6 mM EDTA, 0.01% Tween-20) for 2 min, and then with stringent wash buffer (100 mM MES, 0.1 M NaCl, 0.01% Tween-20) for 1 min. The microarrays were then dipped for a few seconds in final wash buffer (0.1×SSC), and then dried with a microarray centrifuge. Arrays were scanned with a Molecular Devices GenePix 4400A at a resolution of 2.5 μm.
To test the possibility of eliminating reflected light reaching the synthesis area, a small piece of radiochromic film (Far West Technology, FWT-60-20f), with a 2 mm punched hole, was placed in the reaction cell. A 9.5 mm metal disk with a 1 mm pinhole (Edmund Optics, 39730) was aligned over the hole in the film to serve as a physical mask. The entire reaction cell assembly was tilted by ˜7° to move the reflection spot away from the mask hole. The lower chamber was filled with either DMSO (control) or UV absorbers dissolved in DMSO or dichloromethane. The UV absorbers (beta carotene, 9-methylanthracene and riboflavin) were chosen for high extinction coefficients near 356 nm, high Stokes shift, low fluorescence quantum yield and solubility in DMSO. The synthesis cell was exposed using all mirrors, with an exposure of 60 J/cm2 (80 mW/cm2 for 750 s).
Simultaneous synthesis of mirror-image microarrays in this microfluidic photochemical reaction chamber produces high-quality microarrays with little additional cost or effort beyond those of the single microarray synthesis of the legacy method. The results of one such experiment is shown in
The use of a light-absorbing fluid in the lower chamber resulted in the complete blockage of reflected light. Initial trials with 9-methylanthracence and riboflavin in DMSO were only partially successful due to incomplete absorption of violet light from the mercury lamp. Most of the photodeprotection of NPPOC results from the 365 nm line, but the mercury lines at 405 and 436nm are also transmitted through the optical system and result in measureable deprotection. Beta carotene was able to completely absorb the incident light and prevent any reflection. Beta carotene is insufficiently soluble in DMSO, but is highly soluble in dichloromethane, which also has an index of refraction similar to that of glass. The effect of 5.5 mM beta carotene in dichloromethane was tested in comparison to a control experiment with DMSO in the lower chamber. The control showed the reflection from the light transmitted through the 1 mm pinhole as a round exposed spot besides the pinhole dependent on the 7° angle selected according to example 4. Another reflection is also apparent on the left side of the circle; this originates from transmission outside the pinhole disk that is not entirely absorbed by the radiochromic film. The film with the absorbing fluid showed that the beta carotene solution completely suppresses the reflections (
| Number | Date | Country | Kind |
|---|---|---|---|
| 12189396.0 | Oct 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/072070 | 10/22/2013 | WO | 00 |
