Plasma functionalization of AFM tips for measurement of chemical interactions

Abstract

In this paper, a new, fast, reproducible technique for atomic force microscopy (AFM) tips functionalization used for chemical interaction measurements is described. Precisely, the deposition of an aminated precursor is performed through plasma-enhanced chemical vapor deposition (PECVD) in order to create amine functional groups on the AFM tip and cantilever. The advantages of the precursor, aminopropyltriethoxysilane (APTES), were recently demonstrated for amine layer formation through PECVD deposition on polymeric surfaces. We extended this procedure to functionalize AFM probes. Titration force spectroscopy highlights the successful functionalization of AFM tips as well as their stability and use under different environmental conditions.

Introduction

During the last two decades, scanning probe microscopies (SPM) progressively became essential techniques in a variety of domains, ranging not only from chemistry [1], surface physics [2], and biology [3] but also to food and agricultural research [4], biomedical and biosensor devices [5], and the pharmaceutical industry [6]. Originally composed of two members, i.e. scanning tunneling microscope (STM) and atomic force microscope (AFM) [7], the SPM family rapidly extended through the development of AFM derived techniques such as among others: lateral force microscopy (LFM) [8], chemical force microscopy (CFM) [9], electric and magnetic force microscopy (EFM and MFM) [10], force spectroscopy (FS) [11], kelvin probe force microscopy (KFM) [12], and molecular recognition force microscopy (MRFM) [13], [14].

Some of those techniques are providing chemical and biological information through judicious use of chemical and biological interactions. For example, appropriate functionalization of STM tips was previously used to identify functional groups inside molecules organized in self-assembled monolayers at liquid–solid interfaces [15]. Similar techniques discriminate bases in DNA molecules [16]. In atomic force microscopy derivatives such probe modifications were extensively used to identify and quantify chemical and biological interactions [17]. For example, chemical force microscopy reveals specific chemical functionality at the nanoscale by using chemically modified AFM probes [18]. In such a case, hydrogen-bonding and/or hydrophobic interactions are governing the probe–surface interaction, providing chemical contrast in the resulting SPM images. In molecular recognition force microscopy, attachment of antibody at the tip apex allowed the identification and localization of recognition events between antibody on the probe and antigens deposited at surfaces [14].

Generally, those AFM probe modifications require the presence of functional groups like carboxylic or amino groups at the tip apex. The attachment of alkanethiol self-assembled monolayers (SAMs) onto gold (or platinum)-coated AFM probes is the most commonly used probe modification technique [19]. However, this latter procedure is detrimental to some important experimental parameters such as the tip radius of curvature, for example. Moreover, alkanethiol SAM formation on gold-coated AFM probes is mostly performed in the liquid phase. Such procedure may use hazardous materials producing liquid wastes environmentally nonfriendly. Also, these are relatively time-consuming. Finally, SAMs stability may reduce the probe lifetime. Organosilane functionalization of silicon or silicon nitride tips is another possibility, although not really explored [20]. This can originate from the fact that liquid-phase deposition of silane is well known to produce a fairly thick polymeric layer.

In such cases, the gas-phase deposition process can be a good alternative. Although not exposed in the literature, it resolves major issues arising from the liquid-phase deposition process. Particularly, plasma-enhanced chemical vapor deposition (PECVD) retained our attention. Indeed, it is a versatile surface engineering technique which has proven to be an excellent tool for surface modification and large-scale industrial production [21]. Moreover, it enables rapid, low-temperature deposition of various functional groups in a controlled way on a large variety of substrates including complex three-dimensional structures [22]. In this technique, a plasma discharge is created in the presence of a precursor containing the required chemical groups. The deposited coatings are uniformly dispersed, with a precise control on thickness.

This technique has previously been used to modify substrates such as glass, silicon, and polymers in order to monitor their surface properties. For example, hexamethyldisiloxane (HMDSO) PECVD-deposited coatings were used to tailor the hydrophilic properties of polymeric surfaces [23]. Also, amino-functionalization of surfaces has previously been demonstrated using PECVD from ethylenediamine (EDA) or allylamine precursor on glass, silicon, or polypropylene [24], [25]. Recently, an alternative precursor, (3-aminopropyl)triethoxysilane (APTES), was highlighted as appropriate for cycloolefin copolymers amination [26], [27]. Due to the presence of Si–O groups this molecule can create chemical bonding to plasma-activated surfaces, allowing a good adhesion, stability, and high amino content of the coating.

However, up to now, PECVD deposition of aminated precursors on AFM probes in order to functionalize them for chemical applications (as CFM or FS) was not explored. In this paper, we propose to fill this gap by demonstrating the amino-functionalization of AFM probes through PECVD deposition of APTES precursors. The coating is first characterized using X-ray photoelectron spectroscopy (XPS), ellipsometry, dual polarization interferometry (DPI), water contact-angle (WCA) measurements, and AFM imaging. The probe functionalization and coating stability are analyzed through chemical force spectroscopy and chemical force titration measurements.