Elsevier

Ultramicroscopy

Volume 86, Issues 1–2, January 2001, Pages 167-173
Ultramicroscopy

Chemical sensors and biosensors in liquid environment based on microcantilevers with amplified quality factor

https://doi.org/10.1016/S0304-3991(00)00082-6Get rights and content

Abstract

A new technique is presented for bio/chemical sensors, based on microcantilevers, for detection in liquid environment. The low quality factor of the cantilever in liquid is increased up to three orders of magnitude by using Q-control. This enables AC detection that is immune to the long-term drift of the DC cantilever response in liquids, and to temperature variations. This technique has been applied for the detection of ethanol in aqueous solution by using the microbalance method, and for antibody/antigen recognition by the surface stress method. The results show the feasibility and very high sensitivity of these novel devices.

Introduction

A chemical sensor is an analytical device that combines the specificity of a sensing surface to a target substance with a transducer that produces a signal proportional to the target concentration [1]. In the biosensor devices, the target is a biological substance, and the sensing surface is composed of its specific receptor biomolecule. In a similar way, in an atomic force microscope (AFM), a micrometer-sized lever (transducer) bends as a consequence of the interatomic force between a very sharp tip (sensing surface) and a sample (target) [2]. The cantilever deflection is measured with sub-angstrom resolution by a detector that typically consists of a laser beam reflecting off the cantilever into a segmented photodiode. Thus, forces of the order of 10 pN can be measured. AFM provides the topography of organic and inorganic surfaces at atomic and nanometer scale in air, ultra-high vacuum and in liquids. The large expansion of AFM techniques has widely benefited from the commercialization of microfabricated wafers composed of cantilevers of silicon or silicon nitride. This has brought about the birth of a new class of chemical sensors and biosensors based on microcantilevers [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].

In sensors based on microcantilevers, the cantilever is coated with a sensing film or receptor molecules. As a gaseous or liquid sample solution is flowed over the cantilever, the target molecules attach to the sensitized surface. The cantilever itself works like a transducer, thus in the microbalance method, the target substance is detected by a change of the cantilever mass, that gives rise to a change in the resonant frequency of the cantilever (AC detection). In the surface stress method, only one side of the cantilever is coated with the sensing molecules. The surface stress of this side changes as the target substance adheres to the surface, producing a cantilever bending as the surface of this side expands or contracts to balance the surface energy change. The bending is measured by detecting the cantilever deflection (DC detection).

Sensors based on microcantilevers have been mainly developed for detection in a gas environment, giving a resolution in mass of ∼1 pg [6], and a resolution in surface stress of ∼1 mJ/m2 [7], [9], [13]. However, a biosensor detects the biological target in aqueous solutions; there are also environmental, industrial or clinical applications that require chemical sensors that can detect substances in a liquid environment. The development of bio/chemical cantilever sensors for detection in liquid presents two drawbacks: (i) long-term drift of the cantilever deflection [9], [13] and (ii) the quality factor of the cantilever is reduced by about two orders of magnitude when immersed in a liquid [14].

So far, bio/chemical sensors based on microcantilevers for detection in liquid have used the surface stress method where the deflection signal is monitored [7], [9], [10], [12], [13]. This is because the surface energy is very sensitive to the adsorption of a submonolayer of material, whilst the corresponding change of mass can be negligible. In liquids, the deflection signal drifts of the order of 10 nm/min during the first hours of working. This drift arises from temperature variations and chemical reactions on the cantilever, and is a serious limitation for the development of feasible cantilever sensors with high sensitivity and reproducibility. In fact, Butt has reported experiments with chemical sensors and biosensors in liquids using the surface stress method in which the system needed to equilibrate for 1 or 2 days before doing the experiment [9].

Chen et al. have reported experiments in a gaseous environment where the surface stress produced after the adsorption of the target substance also induces a detectable shift of the resonant frequency [6]. Therefore, AC detection can be implemented in these novel sensors independent of the method used, either the microbalance or surface stress method. AC detection is advantageous as it is insensitive to the drift of the deflection signal as a consequence of the thermal drift of the cantilever and optical system positions. AC detection requires a high quality factor (Q) similar to that in a gas, of ∼100. However, in liquids the quality factor is very low, Q∼1.

Here, we present a novel technique that increases the effective quality factor of the cantilever in a liquid up to three orders of magnitude via positive feedback. This has greatly enhanced the resolution and sensitivity of dynamic force microscopy and force spectroscopy [15], [16]. In addition to the active quality factor control, the resonant frequency of the cantilever is tracked with a phase-locked loop (PLL) arrangement. This technique has been applied to the detection of ethanol and antigens in aqueous solution.

Section snippets

Material and methods

The experiments were performed in the fluid cell of a commercial atomic force microscope (Nanoscope IIIa with Multi Mode head, Veeco Digital Instruments). The cantilever was excited either by mechanical-acoustic excitation or magnetic excitation. The mechanical-acoustic excitation is performed by a piezoelectric actuator at the base of the commercially available fluid cell. The frequency spectrum obtained using mechanical-acoustic excitation can probably be attributed to a convolution of the

Description of the technique

A schematic of the experimental set-up is shown in Fig. 1. The cantilever can be excited by using either mechanical-acoustic excitation or magnetic excitation. For simplicity, Fig. 1 shows only the magnetic excitation method. The device consists of two main parts: (i) a positive feedback loop to increase the effective quality factor [15], [16], [19], [20] and (ii) a PLL to track the resonant frequency of the cantilever. The PLL produces a drive signal of the form F1=F0eiωt and monitors the

Chemical sensor of ethanol using the microbalance method

In this experiment a cantilever was coated with a polymer layer of PMMA that is sensitive to the presence of ethanol in aqueous solution. The resonant frequency shift is related to the change of the effective mass of the cantilever as both sides of the cantilever were coated. Fig. 4a shows the real-time monitoring of the resonant frequency for an exposure of 0.5% ethanol in water. The resulting signal consists of two parts. Firstly, the resonant frequency suddenly drops 50 Hz due to the

Biosensor using the surface stress method

In this experiment, the antibody BRAC30 was covalently immobilized on one side of the cantilever (top side). This antibody is the antigen for the secondary antibody STAR71. When the target antibody (STAR71) is flowed over the cantilever, it binds the receptor molecules attached on one side of the cantilever, changing the surface energy of this side with respect to the other. The area of each side contracts or expands in order to minimize the energy of the system, giving rise to a cantilever

Conclusions

We have presented a new technique for bio/chemical sensors based on microcantilevers in liquids. This technique combines the increase of the effective quality factor by the use of a positive feedback and AC detection of the cantilever response. Q-control enhances the sensitivity of the cantilever response by up to three orders of magnitude, and AC detection makes the response signal immune to the long-term drift and temperature variations. These devices present feasibility, miniaturization,

References (20)

  • J.K Gimzewski et al.

    Chem. Phys. Lett.

    (1994)
  • H.P Lang et al.

    Anal. Chim. Acta

    (1999)
  • A.M Moulin et al.

    Ultramicroscopy

    (2000)
  • H.-J Butt et al.

    Interface Sci.

    (1996)
  • A Boisen et al.

    Ultramicroscopy

    (2000)
  • Z Junhui et al.

    Biotechnol. Adv.

    (1997)
  • D Rugar et al.

    Phys. Today

    (1990)
  • T Thundat et al.

    Appl. Phys. Lett.

    (1994)
  • G.Y Chen et al.

    J. Appl. Phys.

    (1995)
  • D.R Baselt et al.

    Proc. IEEE

    (1997)
There are more references available in the full text version of this article.

Cited by (185)

  • Nanosensors for health care

    2020, Nanosensors for Smart Cities
  • FABRICATION OF HOLLOW COMPOSITE STRUCTURE MICROFLUIDIC DOUBLE-END CLAMPED BEAM

    2023, Proceedings of the Romanian Academy Series A - Mathematics Physics Technical Sciences Information Science
View all citing articles on Scopus
View full text