Patent Grant
8294119
The present invention concerns original electrospray sources, their method of manufacture and their applications.
Electrospraying is the phenomenon that transforms a liquid into a nebulisate under the action of a high voltage (M. CLOUPEAU “Electrohydrodynamic spraying functioning modes: a critical review. Journal of Aerosol Science (1994), 25(6), 1021-1036”). To achieve this, the liquid is conveyed into a capillary and is subjected to a high direct current or alternating current voltage or to a superposition of the two (Z. HUNEITI et al., “The study of AC coupled DC fields on conducting liquid jets”, Journal of Electrostatics (1997), 40 & 41 97-102). At the capillary output, the liquid is nebulised under the action of the voltage. The surface of the meniscus formed by the liquid is stretched to form one or several Taylor cones from which are ejected charged droplets of liquid, which develop to give a gas containing charged particles. The formation of the nebulisate is observed when the electrical forces due to the application of the voltage compensate and exceed the surface tension forces of the liquid on the section of the capillary in the end of said capillary.
The size of the capillary, and more precisely its output orifice, is in direct relation to the flow of liquid coming out of the capillary and the voltage to be applied to observe the phenomenon of nebulisation. Two distinct electrospraying operating conditions exist, which are distinguished by their establishment characteristics:
The application of a voltage having an alternating component allows the stabilisation of the electrospraying process by synchronisation on its own frequency (F. CHARBONNIER et al., “Differentiating between Capillary and Counter Electrode Methods during Electrospray Ionization by Opening the Short Circuit at the Collector”. Analytical Chemistry (1999), 71(8), 1585-1591). The chemical composition of the drops produced by the electrospray phenomenon may be improved in view of its applications by the application of multiple and independent voltages that enable the chemical modification of the species present in the liquid by electrochemistry (see US patent application 2003/0015656; G. J. VAN BERKEL, “Enhanced Study and Control of Analyte Oxidation in Electrospray Using a Thin-Channel, Planar Electrode Emitter”, Analytical Chemistry (2002), 74(19), 5047-5056; G. J. VAN BERKEL et al., “Derivatization for electrospray ionization mass spectrometry. 3. Electrochemically ionizable derivatives”, Analytical Chemistry (1998), 70(8), 1544-1554; F. ZHOU et al. “Electrochemistry Combined Online with Electrospray Mass Spectrometry”, Analytical Chemistry (1995), 67(20), 3643-3649).
The application fields of electrospraying are as follows:
These diverse applications may also be combined with each other.
Usually, the sources used for the nanoelectrospray are in the form of capillaries in glass or in fused silica. They are manufactured by hot drawing or by acid attack of the material in order to produce an output orifice of 1 to 10 μm (M. WILM et al., “Electrospray and Taylor-Cone theory, Dole's beam of macromolecules at last?”, International Journal of Mass Spectrometry and Ion Methods (1994), 136(2-3), 167-180). The electrospray voltage may be applied via an appropriate exterior conductive coating: a metal coating such as gold or an Au/Pd alloy (G. A. VALASKOVIC et al., “Long-lived metalized tips for nanoliter electrospray mass spectrometry”, Journal of the American Society for Mass Spectrometry (1996), 7(12), 1270-1272), silver (Y.-R CHEN et al., “A simple method for manufacture of silver-coated sheathless electrospray emitters”, Rapid Communications in Mass Spectrometry (2003), 17(5), 437-441), a carbon based material (X. ZHU et al., “A Colloidal Graphite-Coated Emitter for Sheathless Capillary Electrophoresis/Nanoelectrospray Ionization Mass Spectrometry”, Analytical Chemistry (2002), 74(20), 5405-5409) or a conductive polymer such as polyaniline (P. A. BIGWARFE et al., “Polyaniline-coated nanoelectrospray emitters: performance characteristics in the negative ion mode”, Rapid Communications in Mass Spectrometry (2002), 16(24), 2266-2272). The electrospray voltage may also be applied via the liquid with the introduction of a metallic wire in the source (K. W. Y. FONG et al., “A novel nonmetallized tip for electrospray mass spectrometry at nanoliter flow rate”, Journal of the American Society for Mass Spectrometry (1999), 10(1), 72-75).
Nevertheless, the devices of the prior art dedicated to nanoelectrospray suffer from several weaknesses (B. FENG et al., “A Simple Nanoelectrospray Arrangement With Controllable Flowrate for Mass Analysis of Submicroliter Protein Samples”, Journal of the American Society for Mass Spectrometry (2000), 11, 94-99):
Thus, standard commercial sources are poorly adapted, firstly to a nebulisation that is controlled, reproducible and of high quality, secondly to the use of robots due to the entirely manual character of their mode of use, and, thirdly, to an integration in a fluidic microsystem, as discussed hereafter.
These drawbacks hamper certain electrospraying application fields that require at the present time a robotisation and an automation of the processes. This is the case of the application fields enumerated above: analysis by mass spectrometry, deposition of drops of calibrated size and writing at a sub-micrometer scale by means of a tip.
The last two decades have witnessed the advent of microfluidics in the fields of chemistry and biology. This sector results in part from the miniaturisation of laboratory tools and thereby the marriage between microtechnology and biology or microtechnology and chemical analysis. Thus, microtechnology techniques are put to profit for the manufacture of integrated Microsystems of characteristic size of the order of a micrometer and which group together a series of rectional and/or analytical, chemical and/or biochemical/biological processes.
The development of microfluidics in the fields of chemistry and biology, where the rapidity and the automation of processes are today required, is explained by:
Microfluidic devices are manufactured by means of microtechnology techniques. A wide range of materials is now available for these microfabrications, a range extending from silicon and quartz (normal materials in microtechnology) to glasses, ceramics and polymer type materials, such as elastomers or plastics. Thus, microfluidics benefit both from:
More precisely, the materials that may be envisaged for technological manufacture applicable to chemistry and biology are (T. McCREEDY, “Manufacture techniques and materials commonly used for the production of microreactors and micro total analytical systems”, TrAC, Trends in Analytical Chemistry (2000), 19(6), 396-401):
In particular, micromanufacturing techniques have been applied to the formation of electrospray sources or of needle type tips with a view to:
Manufacturing electrospray tips by means of microtechnology techniques obey two tendencies:
These microfabricated electrospray devices are based, in the image of fluidic Microsystems, on the use of different types of materials and different types of methods.
According to the first tendency, which aims to produce by technological route a capillary type geometry, one can list the following descriptions:
The second tendency is to machine a tip at the output of a microchannel or to create a tip structure that acts as electrospray source. The angle of the tip structure does not seem to have any influence on the nebulisation phenomenon. According to this second tendency:
All in all, the nebulisation devices detailed above have operating conditions that are not compliant for a small scale nebulisation (dimensions too big, nebulisation voltages too high) and most usually result from very complex manufacturing methods. In addition, the type of structure chosen for these different devices is practically indissociable from the material used for their formation.
For the different devices presented above, the nebulisation voltage is usually applied at the level of the reservoir of the device, if the system includes a reservoir, or, if this is not the case, at the level of the supply of liquid, which is achieved by means of a capillary connected to the device. In this case, either the capillary is conductive (in stainless steel for example), or the connection is based on a metallic connection. However, it has been proposed to integrate, on the nebulisation device, an electrode or conductive zone to which is applied the nebulisation voltage (T. C. ROHNER et al., “Polymer microspray with an integrated thick-film microelectrode”, Analytical Chemistry (2001), 73(22), 5353-5357). This conductive zone is formed on the basis of carbon ink in the example cited.
Finally, the application of these devices is targeted for electrospraying preceding an analysis by mass spectrometry and does not lend itself to another type of application.
Moreover, the devices for depositing calibrated drops stemming from microtechnology are not based on the nebulisation of the solution but on a mechanical effect with the bringing into contact of the tip microfabricated on the deposition surface. Thus:
Finally, molecular writing at around the nanometer scale is principally described with an AFM (Atomic Force Microscopy) tip which is soaked in a chemical solution, in the image of a dip pen (G. AGARWAL et al., “Dip-Pen Nanolithography in Tapping Mode”, Journal of the American Chemical Society (2003), 125(2), 580-583; international patent applications WO-A-03/48314 and WO-A-03/52514; H. ZHANG et al., “Direct-write dip-pen nanolithography of proteins on modified silicon oxide surfaces”, Angewandte Chemie, International Edition (2003), 42(20), 2309-2312; L. FU et al., “Nanopatterning of “Hard” Magnetic Nanostructures via Dip-Pen Nanolithography and a Sol-Based Ink”, Nano Letters (2003), 3(6), 757-760; H. ZHANG et al., “Manufacture of sub-50-nm solid-state nanostructures on the basis of dip-pen nanolithography”, Nano Letters (2003), 3(1), 43-45). The writing then takes place by bringing into contact or after coming together, depending on the mode of use of the selected AFM, of the tip and a smooth surface. The chemical solution may also be a solution that attacks the material on which it is deposited and thus serve for the etching of channels or other structures. The AFM technique has the advantage of high resolution and a very high writing precision. Three operating modes are possible and, depending on the mode chosen, the surface state may be controlled before and after passage of the molecular writing chemical solution. Nevertheless, this technique imposes the use of a heavy, bulky, costly and complex apparatus.
Two molecular writing devices described in the literature may also be cited. They derive from the technique using an AFM tip but are based on the use of a microfabricated tip. The first device (A. LEWIS et al., “Dip pen nanochemistry: Atomic force control of chrome etching”, Applied Physics Letters (1999), 75(17), 2689-2691; H. TAHA et al., “Protein printing with an atomic force sensing nanofountainpen”, Applied Physics Letters (2003), 83(5), 1041-1043), is in the form of a micropipette manufactured by means of microtechnology techniques and in which the tip may have dimensions as small as 3 and 10 nm for its internal and external diameters respectively. This micropipette is nevertheless integrated in an AFM apparatus for its use. The ejection of the solution is here provoked not by a bringing into contact but by applying a pressure on the column of liquid. This device has been tested for its aptitude to deliver etching solutions of a layer of chrome deposited on a glass wafer. The second device (I. W. RANGELOW et al., ““NANOJET”: Tool for the nanomanufacture”, Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures (2001), 19(6), 2723-2726; J. VOIGT et al., “Nanomanufacture with scanning nanonozzle ‘Nanojet’ ”, Microelectronic Engineering (2001), 57-58 1035-1042) consists in tips formed in silicon covered with Cr/Au, having a pyramidal shape and an output orifice of size inferior to 100 nm. This device delivers not a chemical solution as in the previous example, but free radicals in the gas phase produced by a plasma discharge that attacks the material placed opposite the tip. Thus, the device does not consist uniquely in a microfabricated tip but also includes a machinery for producing very reactive species, such as radiofrequency or microwave plasma discharge, which can attack the substrate.
These two examples indeed have a microfabricated tip that replaces the conventional AFM tip, but they do not allow one to do away with the heavy and costly peripheral machinery necessary for their operation. Furthermore, this technique is based on a bringing into contact or quasi-bringing into contact of the tip and the substrate. Consequently, the operating parameters must be very meticulously controlled in order to avoid any deterioration in the surface condition due to too high a force applied at the level of the tip.
The present invention concerns a two dimensional electrospray device having a calligraphic pen type geometry, the tip of which acts as the site for the nebulisation.
The subject of the invention is therefore an electrospray source having a structure comprising at least one flat and thin tip in cantilever in relation to the rest of the structure, said tip being provided with a capillary slot formed through the complete thickness of the tip and which ends at the end of the tip to form the ejection orifice of the electrospray source, the source comprising means of supplying the capillary slot with liquid to be nebulised and means of applying an electrospray voltage to said liquid.
According to an advantageous embodiment, the supply means comprise at least one reservoir in fluidic communication with the capillary slot.
Preferably, the structure comprises a support and a wafer integral with the support and in which a part constitutes said tip. The supply means may comprise a reservoir constituted by a recess formed in said wafer and in fluidic communication with the capillary slot.
The means of application of an electrospray voltage may comprise at least one electrode arranged so as to be in contact with said liquid to be nebulised.
In the case where the structure comprises a support and a wafer integral with the support, the means of applying an electrospray voltage may comprise the support, at least partially electrically conductive, and/or the wafer at least partially electrically conductive. Advantageously, the wafer has a surface hydrophobic to the liquid to be nebulised.
The means of applying an electrospray voltage may comprise an electrically conductive wire arranged in order to be able to be in contact with said liquid to be nebulised.
The supply means may comprise a capillary tube. They may comprise a channel formed in a microsystem supporting said structure and in fluidic communication with the capillary slot.
According to an advantageous embodiment, the means of applying the voltage (electrode, support, wafer, wire) also enable the application of the voltages necessary for any device placed upstream in fluidic continuity with the subject of the present invention.
A further subject of the invention is a manufacturing method of a structure being an electrospray source, comprising:
This method may comprise the following steps:
The step of deposition of the wafer may be a deposition of a wafer comprising a recess in fluidic communication with the capillary slot in order to constitute a reservoir. The method may further comprise a step of depositing at least one electrode intended to assure an electrical contact with the liquid to be nebulised.
The electrospray source according to the invention may be used to obtain an ionisation of a liquid by electrospraying before its analysis by mass spectrometry. It can also be used to obtain a production of drops of liquid of calibrated size or the ejection of particles of fixed size. It can also apply to the carrying out of molecular writing by means of chemical compounds. It may also be applied to the definition of electrical junction potential of a device in fluidic continuity.
The invention will be better understood and other advantages and specific features will become clear on reading the description that follows, given by way of non limitative example, with reference to the accompanying drawings, in which:
The present invention draws its inspiration from the structure and the mode of operation of a calligraphic pen. The planar sources that are the subject of the present invention are constituted of the same elements as a calligraphic pen: a liquid reservoir and a two dimensional capillary slot formed in a tip. The present invention may comprise, if necessary, an electrical contact zone to which is applied the voltage necessary for establishing a nebulisate. This contact zone may be structured with multiple and independent contacts and, in particular, three contacts corresponding to a working electrode, also enabling the electrospray voltage to be applied, a reference electrode and a measurement electrode to allow the chemical modification by electrochemistry with a view to favouring the electrospray process or to study it. These electrodes also enable the control of the electrospray process by synchronisation on its own frequency. In the same way that in the calligraphic pen the liquid is conveyed by capillarity in the slot towards the end of the tip of the dip pen type structure where it is ejected. The ejection takes place not by mechanical action, but in the form of nebulisation by application of a high voltage to the liquid.
An electrospray source according to the present invention is represented in
This electrospray source comprises a support 1 and a wafer 2 integral with the support 1. A part of the wafer 2 forms a tip 3 in cantilever in relation to the support 1. The wafer 2 comprises in its centre a recess 4 revealing the surface of the support 1 and constituting a reservoir. A capillary slot 5, also revealing the support 1, connects the reservoir 4 to the end 6 of the tip 3, which forms an ejection orifice for the electrospray source.
The operation of the device is based on the following formulated principles. The reservoir of liquid 4 contains the liquid or serves as transit for the supply with liquid. The liquid is then guided by the capillary slot 5 upstream of which is located the reservoir 4 of liquid. The tip of the structure enables the establishment of an electrospray.
The following mode of operation ensues from this. The liquid of interest is deposited or conveyed into the reservoir of liquid 4 by an appropriate method. It is guided towards the end 6 of the structure by capillarity. The source is brought to its site of use (for example in front of a mass spectrometer). A potential is applied to the liquid so as to observe the nebulisate at the end 6 of the tip.
The physics of the source having a dip pen type geometry is based on the properties of the materials that constitute it and the dimensions of its different elements.
The role of the reservoir 4 is to contain the liquid to be nebulised and to progressively supply the capillary slot 5. The topology of the structure is two dimensional. The wafer 2 is in a material with hydrophobic character, and even more hydrophobic than that constituting the support 1 supporting the wafer 2, material that covers the base of the reservoir. This makes it possible to limit the losses of liquid outside of the reservoir. It is interesting to note in this respect that the liquids envisaged for the nebulisation are a priori of rather hydrophilic character, such as purely aqueous solutions or half-aqueous half-alcoholic solutions, for example 50/50 methanol/water mixtures.
The capillary slot 5 and the end 6 of the tip 3 are formed in the material forming the wafer 2 and their dimensions are determined during the manufacturing method. In
where (r) is the internal radius of the capillary, (hr) the height to which the liquid rises in the capillary tube, (ρ) the density of the liquid, (α) is the contact angle of the liquid on the internal walls of the capillary tube and (g) is the acceleration of gravity.
γ cos α=γSV−γSL (Equation 2)
where γSV is the surface tension at the solid-vapour interface and γSL is the surface tension at the solid-liquid interface.
Firstly, in the case where α<90° (cos α>0), Young's equation (equation 2) implies that γSV>γSL and therefore that the solid-liquid interaction is favoured compared to that of the solid-vapour. The term r appears in equation 1. The observation or not of the capillarity effect depends on its value. The term r corresponds to the radius of the capillary tube and, in the case of the device that is the subject of the present invention, to the dimension of the capillary slot 5. If the liquid penetrates into the capillary slot, a liquid bridge between the two walls of the capillary slot is formed. One may thus define an aspect ratio R for the capillary slot 5, corresponding to the ratio h/w. It ensues from the preceding that R must be greater than a critical value to observe a capillarity effect in the capillary slot 5 and so that the formation of the liquid bridge in the capillary slot 5 is favoured from an energetic point of view.
The nebulisation device may include or not conductive zones (see
Nevertheless, depending on the nature of the material chosen to form the support 1 of the electrospray source, these conductive zones, in particular if their role is to convey the nebulisation voltage, may not be necessary. Indeed, if a conductive material (metal, Si, etc.) is used to form the support 1 or the wafer 2, the voltage will be applied directly to this conductive material. Finally, a device not comprising conductive zones and for which the materials are not conductive may be used in electrospraying provided that the electrical contact is achieved via the liquid. A metallic wire immersed in the solution to be nebulised, at the level of the reservoir 4 or any other conductive contact will thus assure the role of application of the nebulisation voltage.
The device may also be connected to a liquid supply source upstream of the reservoir 4, such as a capillary conveying a solution coming from another apparatus, another structure. For example, for a mass spectrometry type application, the capillary may correspond to a separation column output. For a deposition of drops of calibrated size or molecular writing type application, this capillary conveys the liquid towards the nebulisation device from its initial location. Said capillary may be a conventional commercial capillary in fused silica. It may also be a microfabricated capillary, in other words a microchannel integrated on the system supporting the source. The capillary may be a hydrophilic track materialised on the support 1. In these two latter cases, the wafer 2 is integrated on a fluidic microsystem and plays the role of interface between said microsystem and the exterior world where the solution exiting the microsystem is used. Finally, the conductive properties of the device or one of its elements may be used to electrically supply any system in fluidic relation with the device.
Moreover, said dip pen type wafers may be used in an isolated manner or be integrated in large numbers on a same support, and this with a view to the parallelisation of the nebulisation. In this case, said dip pen type wafers are independent or not of each other and the nebulised solutions are, either the same in order to increase the nebulisation of said solution, or different and, in this case, the dip pens function in a sequential manner in nebulisation. The integration of said dip pen type wafers may be carried out in a linear manner with an alignment of said wafers on a side of the support or in a circular manner on a round support. Going from one source to another is then achieved respectively by translation or by rotation of the support.
A wide range of materials may now be envisaged for microtechnological manufactures and in particular fluidic microsystems: glass, silicon based materials (Si, SiO2, silicon nitride, etc.), quartz, ceramics and a large number of macromolecular materials, plastics or elastomers.
The geometry retained for the present invention is compatible with manufactures using any type of materials, and, for the different parts comprising the electrospray source: the support 1, the dip pen type wafer 2 and the conductive zones. Moreover, the method of technological manufacture involves one or several other material(s), the choice of which is adapted as a function of the materials retained for the elements 1, 2 and 3.
A generic method of manufacturing electrospray sources according to the invention is represented in
The first step of this method of manufacture is the choice of the substrate intended to constitute the support of the electrospray source. This substrate 10 (see
The start of the method conditions the end of the manufacture of the electrospray devices. It involves the materialisation on the support of the device of lines that will aid the cleavage of the substrate in order to free the tip of the source and enable the nebulisation.
According to the second step, a layer 11 of material known as a protection layer is deposited on a part of the substrate 10. The material of the layer 11 is chosen as a function of the nature of the material of the substrate 10 in such a way that an attack of the layer 11 does not affect the substrate 10. In this embodiment, the layer of protective material is a layer of silicon oxide of 20 nm thickness. The layer 11 is of variable thickness depending on the nature of the materials of the substrate 10 and the layer 11. The layer 11 is subjected to a lithography step intended to reveal the zones of the substrate to be attacked to define cleavage lines delimiting the support of the structure. The corresponding zones of the layer 11 are attacked in order to provide openings 12 revealing the substrate 10 (see
During a third step, a layer of sacrificial material is deposited on the substrate 10. This layer of sacrificial material 14 will enable at the end of manufacture the tip of the structure to overhang its support before the cleavage operation. The substrate 10 is covered with a thin film of sacrificial material of sufficient thickness so that, after its elimination, the tip is sufficiently separated from the substrate 10, but nevertheless sufficiently thin in order to do away with any problem of stressing and curving of the tip overhanging the support. In this embodiment, the layer of sacrificial material is a layer of nickel 150 nm thick.
The layer of sacrificial material is then subjected to a lithography step and appropriate attack in order to only retain of this material a zone 14 corresponding to the tip of the structure (see
The fourth step may be implemented. The substrate 10 is then covered with a layer of a material intended to constitute the wafer of the structure. As a function of the material of the substrate, the material of this layer may be silicon or based on silicon, a metal or even a polymer or ceramic type material. In this embodiment, the layer of material intended to constitute the wafer is a layer of 35 μm thickness in SU-8 2035 polymer purchased in pre-polymerised form from Microchem and polymerised by a photolithographic method. The thickness of this layer is chosen in an appropriate manner. Indeed, the ionisation performance of the nebulisation device depends on this thickness, as has been explained previously. The thickness of this layer influences directly the height h of the capillary slot and, according to the preceding, the bigger h is, the bigger w has to be in order not to modify the ratio R. However, depending on the final application of the nebulisation source, the challenge is to reduce was far as possible in order to increase the performance. On the other hand, if the thickness of the layer intended to constitute the wafer is too thin, the overhanging tip may bend once disbanded from the support due to the stresses applied to the material. Those skilled in the art will be capable of adapting the present specification as a function of the nature of the material of this layer and thus define the optimal thickness of material to be deposited.
This layer then undergoes a lithography step and an attack in order to form the dip pen type wafer 2, in other words in addition to its size, the reservoir 4, the capillary slot 5 and the tip 3 (see
The fifth step may then be undertaken. Once the wafer 2 has been formed, the zone 14 of sacrificial material under the tip 3 may be removed. The sacrificial material is removed by a suitable chemical attack. The solution for this chemical attack must be chosen judiciously so that all of the sacrificial material is eliminated without either the support or the wafer being affected. The materials of these elements must not be sensitive to this chemical solution. One obtains the structure shown in
The sixth step concerns the implantation of conductive zones on the structure. As mentioned previously, this step is only included in the method of manufacture if such conductive zones are provided for.
Whether these zones are located at the level of the reservoir 4 (application of the nebulisation voltage) or at the level of the tip (physical/chemical study electrodes), the manufacturing method is the same. The formation of conductive zones 3 at the level of the reservoir alone will be detailed here.
These conductive zones may be in metal or in carbon. The structure is firstly subjected to a masking step so that only the zones corresponding to the formation of conductive zones are cleared. The conductive material chosen is then deposited by a PECVD (Plasma Enhanced Chemical Vapour Deposition) technique on the structure. In this embodiment, the conductive zones are in palladium and have a thickness of 400 nm.
The seventh step of this method of manufacturing the nebulisation source is the detachment of the support 1 in relation to the substrate 10 and, in particular, the placing in cantilever of the tip 3 in relation to the support 1 by using the cleavage lines 13 materialised in the second step of this manufacturing method. The structure obtained is represented in
An advantageous cleavage technique is illustrated in
This generic manufacturing method is then adapted as a function of the materials chosen for each element of the electrospray source.
The first application field targeted by the present invention is the electrospraying of biological or chemical solutions to be analysed by mass spectrometry. Mass spectrometry is at the present time the technique of choice for the analysis, the characterisation and the identification of proteins. However, since the completion of the deciphering of the genome, biologists in particular have become more and more interested in proteomics, a science that aims to study and characterise all of the proteins of an individual. These proteins, in all human beings, are present in numbers of more than 106 different molecules, including post-traductional modifications. This point justifies the need, at the present time, of analysis techniques and tools compatible with an automation with a view to a high rate analysis, and this particularly for mass spectrometry due to its pertinence within the scope of the study of proteins. The samples (or solutions to be analysed) that are available to the biologist are often of restricted size (less than or equal to 1 μL) and contain little biological material, which imposes working with a very sensitive analysis technique and consuming little of the sample. This makes mass spectrometry with an ionisation by nanoelectrospray one of the most widely used analysis techniques for the characterisation of proteins. In this context, the major challenge is the reduction, as far as possible, of the dimensions of the end of the tip of the source. Indeed, as mentioned in the introduction, two electrospray operating conditions for this type of application, the most interesting in terms of automation and gain in sensitivity being the nanoelectrospray operating condition. However, at the present time, the analysis speed is limited, the flow rate of samples restricted due to the fact that the nanoESI-MS (for “nano ElectroSpray Ionization-Mass Spectrometry”) is entirely based on manual processes. The tools presently available do not lend themselves to a robotised and automated analysis. This context explains the motivations for the development of the present invention for this type of application.
The second type of application targeted by the present invention is the deposition of calibrated drops on a smooth or rough surface. This is of prime interest for the preparation of DNA, peptide and PNA chips or any other type of molecule. This type of application requires a device capable of conveying the fluid in discrete form, of drops of liquid of calibrated size, the size usually depending on the desired resolution in the preparation of the analysis wafers. The smaller the drops, the more their deposition on the wafer can be closer together and the higher the density of deposition and therefore the higher the density in substances to be analysed. The device that is the subject of the present invention may be used for this purpose. The width of the capillary slot 5, and the value of the applied voltage for the ejection of the drops conditions the size of the drops ejected by said nebulisation device. Thus the resolution of the analysis wafers may be adjusted as a function of the width of the slot of the device. Finally, the nebulisation voltage may be alternating and thus give a rate of deposition in drops/minute depending directly on the frequency of the alternating voltage. The deposition of calibrated drops as presented above may be used for the preparation of analysis wafers such as DNA chips. It may also be applied to the preparation of MALDI targets (for “Matrix-Assisted Laser Desorption/Ionization”) on which the samples to be analysed by mass spectrometry with a MALDI ionisation here, are deposited in a discrete manner before their crystallisation and their introduction into the mass spectrometer. Thus, the present nebulisation device having a dip pen type geometry may be for example connected to a separation column output and enable a coupling between a separative technique and an in line MALDI type analysis by mass spectrometry. The drops of liquid finally may be replaced by cells. In this case, the cells are similarly ejected in a discrete manner and deposited for example on a wafer with a view to the elaboration of cell chips.
The third application targeted by the present invention is molecular writing at scales of around one hundred nanometers. At the present time, this type of operation is carried out by means of AFM tips, functioning by means of a heavy and bulky apparatus. The ejection of the liquid is based on a bringing into contact or quasi-contact of the tip and the deposition substrate in the case of AFM or on the application of a pressure on the liquid. An adaptation of this technique is to eject the liquid under the action of a voltage and not by means of a pressure or a bringing into contact. Indeed, in both cases, the ejection is induced when the tension forces of the liquid at the level of the tip of the pipette are “exceeded” by another force applied to the column of liquid. This may be envisaged with an electrospray device where the electrical force exceeds that of the liquid tension and thus leads to the formation of droplets. Furthermore, the formation of reactive species is intrinsic to the electrospray process. This fluid ejection technique does away with any complex apparatus for producing reactive species such as free radicals, such as a plasma or microwave discharge, upstream of the structure that delivers the liquid.
The present invention may therefore be used for such writing purposes on a smooth or rough substrate, the liberation of the writing solution (pseudo-ink) here being governed by application of a voltage. In the same way as for the first application field, a major challenge is to minimise the size of the end of the tip, this dimension conditioning the size of the ejections by nebulisation and consequently the desired writing resolution on the final substrate. The width of the tip is less than or equal to a micrometer. Another factor influencing the size of the ejections and the fluid flow rate is the nebulisation voltage applied to the liquid. Finally, the production of reactive species, if the device is used to dispense a solution for attacking the substrate, may be enhanced with the implantation of electrodes within the dip pen type structure that conveys the fluid. These electrodes are then the site of electrochemical reactions leading to the formation of reactive species We will now interest ourselves in the following examples.
A first example concerns the dimensions and the shapes chosen to form a nebulisation device as described in the present invention.
This first device has small tip dimensions due to the targeted application field, in other words a nanoelectrospray for the ionisation of solutions before their analysis by mass spectrometry. The device is formed in accordance with
The second example concerns the manufacture by microtechnology of nebulisation sources, as described in example 1. The materials used are silicon for the support 1 and the negative photolithographic resin SU-8 for the dip pen type wafer 2. The method of manufacture stems from the method described above. It is adapted to the materials chosen.
A substrate of silicon oriented (100) and n doped, of 3 inches, is covered with a layer of 200 nm of silicon oxide (SiO2), then masked by lithography. The layer of SiO2 is attacked by an acid solution of HF:H2O on the non-masked zones. The exposed silicon is then attacked by a caustic soda solution (KOH) so as to materialise the cleavage lines. A layer of 150 nm of nickel is then deposited on the silicon surface by a spraying technique under argon (Plassys MP 450S). The layer of nickel is attacked in a local manner by UV photolithography (positive photosensitive resin AZ1518 [1.2 μm], etching solution HNO3/H2O (1:3)) so that nickel only remains under the tip of the dip pen. After elimination of any trace of photolithographic resin, the wafer of silicon is dehydrated at 170° C. for 30 min, so as to optimise the adhesion of the resin SU-8 on the silicon surface. A layer of 35 μm of resin SU-8 is spread out on the silicon substrate by means of a whirler to homogenise the thickness before the following step of photolithography. The dip pen type wafer 2 is formed in this layer of resin SU-8 by means of conventional UV photolithography techniques. After development of the resin SU-8 with the appropriate reagent (1-methoxy-2-propanol acetate, PGMEA), the layer of nickel is attacked with the acid solution (HNO3/H2O) described above. This step of chemical attack of the nickel does not affect the resin SU-8 even if this method can take several hours. Finally, after drying of the device, the silicon substrate 1 is sawed according to the technique illustrated in
The method of manufacture described above does not include the formation of electrodes.
A third example concerns the dimensions and the shapes chosen for forming a particle ejection device having a size of around one hundred micrometers, as described in the present invention.
This device has larger dimensions than that described in example 1. Here, the dimensions of the capillary slot 5 and the reservoir 4 must be compatible with the handling of objects of around one hundred micrometers. Due to this range of dimensions, the device described in example 3 also applies to the handling of cells of size close to 100 μm diameter, for the preparation of cell chips for example.
The reservoir 4 of said device has for dimensions 1 cm×1 cm×e (μm), where e is the thickness of the wafer 2. In the same way as example 1, the value of e is defined as a function of the width of the capillary slot 5 so as to have an aspect ratio R in the end 6 of the wafer that is greater than 1. The particles handled by this device have a size of around one hundred micrometers, therefore the capillary slot 5 has to have a width greater than 100 μm. However, since the particles may have a tendency to aggregate, this width must not be chosen too large. It is preferably close to double the size of the particles handled. As a result, the width of the slot is fixed at 150 μm, and the thickness of the wafer at 200 μm.
The material retained for the manufacture of the dip pen type wafer 2 is here again the negative photolithographic resin SU-8 and the material chosen for the support 1 is glass. The resin SU-8 is interesting here for handling particles such as cells, because these cells do not adhere to this material. As a result, the support 1 in glass is itself also covered with a thin film of resin SU-8 in order to prevent any non desired adhesion of cells on the device.
Example 4 is the test of nebulisation sources manufactured as described in example 2 for a mass spectrometry analysis. In this first example, the nebulisation voltage is applied to the liquid to be nebulised by means of a platinum wire immersed in the liquid at the level of the reservoir as illustrated in
The nebulisation device is placed on a mobile part 30 that can be displaced in xyz. This mobile part 30 comprises a metallic part 31 to which is applied the ionisation voltage in the mass spectrometer 25. The silicon support 1 is isolated as a precautionary measure from this metallic part 31 during the fixation of the device on said mobile part 30 due to the semi-conductive properties of this material. The electrical contact between the metallic part 31 and the reservoir of the device is assured by means of a platinum wire 32 introduced in the reservoir and which is immersed in the solution to be analysed 33. The solution used for the nebulisation tests, a solution of standard peptide (Gramicidine S), is deposited in the reservoir of the device and the mobile part 30 is introduced in the input of the mass spectrometer 25. The tests are carried out on a from Thermo Finnigan ion trap type mass spectrometer (LCQ DECA XP+). The voltage is then applied to the liquid. A camera installed on the ion trap enables the Taylor cone to be visualised, once the voltage is applied. The capillary slot has a width of 8 μm.
Example 5 is similar to example 4, but here the voltage is not applied by means of a platinum wire but by exploiting the semi-conductive properties of silicon.
Example 5 is therefore the test by mass spectrometry of nebulisation sources manufactured according to example 2 with an application of the ionisation voltage to the material constituting the support 1 of the nebulisation device.
In the same way as previously, the nebulisation device is fixed on a mobile part 40 that can be displaced in xyz and having a metallic part 41. Here, the silicon support 1 is brought into electrical contact with the metallic part 41 of the mobile part 40 to which is applied the ionisation voltage in the mass spectrometer 25. The device is fixed on the mobile part 40 by means of a Teflon tape, which surrounds the device upstream of the reservoir. The test is conducted as previously after introduction of the mobile part 40 in the ion trap 25 and application of the voltage. The capillary slot has a width of 8 μm.
The tests were conducted with another standard peptide, Glu-Fibrinopeptide B. The ionisation voltages, here, are in the same range as previously, from 1 to 1.4 kV for peptide concentrations less than 1 μM.
Example 6 is identical to example 5 as regards the manner of conducting the test. The test assembly is identical to that of the previous example, the nebulisation device corresponds to that described in example 1 and carried out according to the method of manufacture described in example 2. The voltage is applied directly to the material of the support 1, silicon, via the metallic zone 41 included on the mobile part 40 introduced in the mass spectrometer 25 (see
The solution is the same as previously, a solution of standard peptide, Glu-Fibrinopeptide B at concentrations less than or equal to 1 μM. Here, the peptide is subjected to a fragmentation experiment. The peptide in double charged form (M+2H)2+ is specifically isolated in the ion trap and is fragmented (standardised collision energy parameter of 30%, radiofrequency activation factor set at 0.25).
Example 7 is identical to example 5 (same device manufactured according to the same method and tested under the same conditions with application of the voltage to the silicon support 1) except that the sample analysed here is no longer a standard peptide but a complex mixture of peptides obtained by digestion of a protein, Cytochrome C. This digestate is composed of 13 peptides of different lengths and physical/chemical properties. This digestate is tested at a concentration of 1 μM and with an ionisation voltage of 1.1-1.2 kV. The width of the capillary slot is 8 μm.
Example 8 is identical to example 5 (same device manufactured according to the same method and tested under the same conditions with application of the voltage to the silicon support 1) except that the sample analysed here is continuously conveyed to said device by a capillary connected to a syringe pump or a nanoLC chain upstream.
For the coupling to a syringe pump, the flow of liquid has been fixed at 500 mL/min. The solution for this test is identical to that of example 5, except that the concentration of the peptide Glu-Fibrinopeptide B is here 1 μM and the nebulisation voltage has been set at 1.2 kV. The width of the capillary slot is 8 μm.
The coupling to a nanoLC chain (liquid chromatography at a flow rate of 1 to 1000 nL/min) has been carried out with conventional conditions of coupling between a separation on nanoLC and an in line analysis by mass spectrometry on an ion trap. The fluid flow rate is 100 nL/min, the ionisation 1.5 kV. The separation experiment is carried out on a digestate of Cytochrome C at 800 fmol/μL and 800 fmol of this digestate are injected in the separation column. The width of the capillary slot is 10 μm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03 50820 | Nov 2003 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2004/050580 | 11/10/2004 | WO | 00 | 3/9/2007 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2005/046881 | 5/26/2005 | WO | A |
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| 5165601 | Rodenberger et al. | Nov 1992 | A |
| 5994696 | Tai et al. | Nov 1999 | A |
| 6602472 | Zimmermann et al. | Aug 2003 | B1 |
| 6633031 | Schultz et al. | Oct 2003 | B1 |
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|---|---|---|
| B-S63-46139 | Dec 1988 | JP |
| 2001-343361 | Dec 2001 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20070252083 A1 | Nov 2007 | US |
