Microfabricated diffusion-based chemical sensor

Abstract

A channel-cell system is provided for detecting the presence and / or measuring the presence of analyte particles in a sample stream comprising: a) a laminar flow channel; b) two inlet means in fluid connection with said laminar flow channel for respectively conducting into said laminar flow channel (1) an indicator stream which may comprise an indicator substance which indicates the presence of said analyte particles by a detectable change in property when contacted with said analyte particles, and (2) said sample stream; c) wherein said laminar flow channel has a depth sufficiently small to allow laminar flow of said streams and a length sufficient to allow particles of said analyte to diffuse into said indicator stream to the substantial exclusion of said larger particles in said sample stream to form a detection area; and d) outlet means for conducting said streams out of said laminar flow channel to form a single mixed stream.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. application Ser. No. 09 / 426,683 filed Oct. 25, 1999, which is a continuation of U.S. application Ser. No. 08 / 829,679 filed Mar. 31, 1997, now U.S. Pat. No. 5,972,710 issued Oct. 26, 1999, which is a continuation-in-part of U.S. application Ser. No. 08 / 625,808 filed Mar. 29, 1996, now U.S. Pat. No. 5,716,852 issued Feb. 10, 1998. This application also claims priority to U.S. application Ser. No. 09 / 703,764 filed Nov. 1, 2000, which is a continuation-in-part of co-pending application Ser. No. 09 / 500,398, filed Feb. 8, 2000, a continuation of application Ser. No. 09 / 346,852 filed Jul. 2, 1999, which is a divisional application of application Ser. No. 08 / 663,916 filed Jun. 14, 1996, now U.S. Pat. No. 5,932,100 issued Aug. 3, 1999, claiming priority to application No. 60 / 000,261 filed Jun. 16, 1995, all of the foregoing applications being incorporated herein by reference to the extent not inconsiste...

AI Technical Summary

Benefits of technology

[0028] The laminar flow channel is long enough to permit small analyte particles to diffuse from the sample stream and have a detectable effect on an indicator substance or detection means, preferably at least about 2 mm long. The length of the flow channel depends on its geometry. The flow channel can be straight or curved in any of a number of ways. In one embodiment, the flow channel can include one or more “hairpin turns,” making a tight stairstep geometry. In another embodiment, the flow channel can be in the shape of a coil, like a neatly wound up garden hose. Non-straight channel geometries allow for increasing the length of the flow channel without increasing the size / diameter of the substrate plate in which the channel is formed, e.g., a silicon microchip. The diffusion coefficient of the analyte, which is usually inversely proportional to the size of the analyte, affects the desired flow channel length. For a given flow speed, particles with smaller diffusion coefficients require a longer flow channel to have time to diffuse into the indicator stream.

[0029] Alternatively, to allow more time for diffusion to occur, the flow rate can be decreased. However, several factors limit the minimum flow rate and therefore make a longer flow channel desirable in some cases. First, the flow rate is achieved by a pumping means or pressure source, some of which cannot produce as low a pressure and flow rate as may be desired, to allow enough time for diffusion of particles with small diffusion coefficients. Second, if the flow rate is slow enough and some particles are of significantly different density from the surrounding fluid streams, particles denser than the surrounding fluid streams may sink to the bottom of the flow channel and particles less dense than the surrounding fluid streams may float to the top of the flow channel. It is preferable that the flow rate be fast enough that hydrodynamic forces substantially prevent particles from sticking to the bottom, top, or walls of the flow channel. Third, a small change in pressure leads to larger errors in measurement accuracy at lower flow rates. Fourth, at low flow rates, other factors, such as changes in viscosity of fluids, can lead to larger errors in measurement accuracy.

[0030] The flow channel can be straight or non-straight, i.e., convoluted. A convoluted flow channel as used herein refers to a flow channel which is not straight. A convoluted channel can be, for example, coiled in a spiral shape or comprise one or a plurality of “hairpin” curves, yielding a square wave shape. Convoluted channels provide longer distances for diffusion to occur, thereby allowing for measurement of analytes with larger diffusion coefficients, e.g., typically larger analytes. In preferred embodiments of this invention wherein a silicon microchip is the substrate plate in which the flow channel is formed, the channel length of a straight flow channel is between about 5 mm and about 50 mm. In preferred embodiments of this invention wherein the flow channel is convoluted, i.e., non-straight, the length of the flow channel is defined or limited only by the size of the microchip or other substrate plate into which the channel is etched or otherwise formed. The channel width (diffusion direction) is preferably between about 20 micrometers and about 1 mm. The channel is more preferably made relatively wide, e.g. at least about 200 micrometers, which makes it easier to measure indicator fluorescence with simple optics, and less likely for particles to clog the channel. However, the channel can be made as narrow as possible while avoiding clogging the channel with the particles being used. Narrowing the width of the channel makes diffusion occur more rapidly, and thus detection can be done more rapidly. The channel depth is small enough to allow laminar flow of two streams therein, preferably no greater than about 1000 micrometers and more preferably between about 50 micrometers and about 400 micrometers.

[0049] The method and system of this invention include determining the concentration of the analyte particles in the sample stream by detecting the position within the laminar flow channel of analyte particles from the sample stream diffusing into the indicator stream causing a detectable change in the indicator stream or in an indicator substance in the indicator stream. The sample stream and the indicator stream may be allowed to reach equilibrium within the laminar flow channel. The location of the boundary of the detection area (i.e. that portion of the indicator stream containing diffused particles at a detectable concentration) with the unaffected indicator stream may be used to provide information about flow speed and / or sample concentration. The physical location of this boundary in the channel for a given analyte stays the same over time as long as the flow speed is constant and the sample unchanged. The location and size of the detection area can be varied by varying flow rate, sample concentration, and / or concentration of an indicator substance so as to optimize the signal for detection.

[0052] Advantages of this system include the fact that analytes can be determined optically in turbid and strongly colored solutions such as blood, without the need for prior filtering or centrifugation; cross-sensitivities of indicator dyes to larger sample components (a common problem) can be avoided; and the indicator can be kept in a solution in which it displays its optimal characteristics (e.g., cross-sensitivities to pH or ionic strength can be suppressed by using strongly buffered solutions). Measurements of the indicator stream at several locations along the channel can compensate for some remaining cross-sensitivities. In addition, the flow channel can be wide, which makes it easy to measure the indicator fluorescence with simple optics. No membrane is needed; the system is less subject to biofouling and clogging than membrane systems. The system is also tunable in that sample or indicator stream concentrations and / or flow rates can be varied to optimize the signal being detected. For example, if a reaction takes about five seconds, the system can be adjusted so that the reaction will be seen in the central portion of the device.

[0053] The method can be conducted by a continuous flow-through of sample and indicator streams. The steady-state nature of this method makes longer signal integration times possible.

Problems solved by technology

None of the foregoing publications describe a channel system capable of analyzing small particles in very small quantities of sample containing larger particles, particularly larger particles capable of affecting the indicator used for the analysis.

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