Layer-by-Layer Assembly of Heterogeneous Modular Nanocomposites

Abstract

Materials and MethodsGold nanoparticles. The gold colloid dispersion was prepared as previously described in the literature (ref 17). Briefly, a 100 mL of stock Au(III) aqueous solution was prepared from 1.00 g tetrachloroauric acid (HAuCl 4 , ≥99.9% purity, Aldrich) in an amber volumetric flask. The stock solution then was aged for one month in the dark.Before preparing the Au colloid, an Ehrlenmeyer flask and a Teflon® stirbar were cleaned by soaking in aqua regia (3:1 mixture of HCl and HNO 3 ) for 20-30 min, followed by a triple rinse in deionized water, and drying overnight in an oven at 110°C. Caution: Aqua regia solutions are extremely corrosive and may result in explosion or skin burns if not handled properly. A mixture of 1 mL of the stock Au(III) solution and 100 mL of deionized water was added to the clean flask, the flask then was covered loosely with a watch glass and the stirred contents were brought to a gentle boil. A 2.5 mL aliquot of an aqueous solution containing 1% wt. trisodium citrate (99%, Aldrich) was quickly added to the stirred contents of the flask. After formation of a clear, cherry red sol, it was gently boiled for an additional 10 min to complete the reduction, then allowed to cool to room temperature covered with the watch glass. The stirbar was removed, deionized water was added to return the volume to 100 mL, and the flask was capped tightly and stored for later use.Substrate cleaning. Two types of substrates were used for LbL assembly: polished fused silica slides (Dell Optics, Inc.) and single-side-polished Si wafers (n-type, As-doped, <100> orientation, Recticon Corp.) For initial cleaning, Si wafers and fused silica slides were placed in a slotted Coplin jar and covered with a 1:1 v/v mixture of methanol and concentrated HCl. The jar was loosely capped and allowed to stand for 45 min in a fume hood. Then the cleaning solution was drained and the substrates were thoroughly rinsed with deionized water. After draining the water, the substrates were covered with concentrated H 2 SO 4 and allowed to stand for 45 min in the loosely capped jar. The sulfuric acid then was decanted away and the substrates were thoroughly rinsed with deionized water.Substrate modification. After cleaning, the substrates were modified by chemisorption of EDA (N-(2-aminoethyl)-N-3aminopropyltrimethoxysilane, Gelest, Inc.) The EDA solution was prepared by adding 2 mL of EDA to 200 mL of deionized water, mixing to dissolve the EDA, and then adding 240 µL of glacial acetic acid. The clean substrates were covered with the EDA solution for 30 min at room temperature then thoroughly rinsed with deionized water. Immediately after the final rinse, the substrates were dried in a filtered N 2 gas stream, baked 6-8 min in an oven at 110-120°C, cooled, and stored in plastic containers.Polyelectrolyte solutions. A stock solution of 1.00 M NaCl (aq) was prepared in a volumetric flask by dissolving 58.

Divalent–Anion Salt Effects in Polyelectrolyte Multilayer Depositions

“…The XPS characterizations of the PEMLs on Si-EDA substrates were performed using the instrument and procedures described previously. 7 FTIR spectra were acquired on a Nicolet FTIR spectrometer coupled to a single-bounce attenuated total reflectance (ATR) accessory (Specac Silvergate) with a 45°Ge prism and 7 mm sample window. Spectra were collected UV−vis spectra of quartz slides bearing PEML films were acquired using a Cary 5000 UV−vis-NIR double-beam spectrophotometer equipped with Cary WinUV software.…”

“…4,10−16 Characteristics of the PEMLs, such as thickness, roughness, and structure, are determined by factors including the concentration, molecular weight, and chemical nature of polyelectrolytes, as well as the solvent, pH, ionic strength, and temperature during film deposition. 10,12,17−23 PEML properties are also influenced by the polyelectrolyte deposition method (e.g., spray-coating, 5,24−26 spin-coating, 17,27 or dip-coating 24,25 ), the rinsing protocol, 7,28,29 and any cross-linking groups. 14,30 The ionic strength of deposition solutions is critically important in determining the properties of PEMLs.…”

“…57 We have recently observed the variation in film thickness and morphology of heterocomposite PEMLs (comprising PAH, PSS, and colloidal Au) as a function of the salt added to polyelectrolyte deposition solutions and of the rinsing protocol. 6,7 Using SO 4 2− as a model dianion, here we systematically investigate the effects of multivalent anions on PEML fabrication. From the dependence of the UV absorbance, thickness, roughness, and structure of deposited PSS/PAH multilayers on the solution SO 4 2− concentration, [SO 4 2− ], we derive a generic anion-bridging model for the polyelectrolyte deposition mechanism.…”

Optical interference effect corrections for absorbance spectra of layer-by-layer thin films bearing covalently bound dye

“…Polished quartz slides (Quartz Scientific Inc., 25.4 mm x 50.8 mm  1 mm) were cleaned [28] and silanized with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA) [29] per the literature procedures. Preparation procedures for the various salt solutions used for PEML deposition (i.e., $3 mg RuPAH mL À1 (vide infra) and 2 mg PSS mL À1 ) and the salt rinse protocol used to deposit PEMLs onto the EDA-coated quartz (i.e., Q-EDA) have been described in detail elsewhere [17,20]. During PEML deposition any substrates bearing partially complete films were stored in PSS solution overnight (i.e., $16 h) when necessary, before deposition was resumed the next day.…”

“…PEMLs so produced have already been applied to areas of metal enhanced fluorescence [7], photovoltaic sensitization [8], light-emitting diodes [9], permselective membranes [3], heterogeneous catalysts [10,11], sensors [12,13], microcapsules [3,14], and pH controlled release delivery systems [15,16]. A valuable attribute of LbL deposition is control over the PEML physicochemical properties by variation of factors such as pH [15,16], the presence of salts [2,17] (as well as ionic strength [18]), and temperature [19] to tune layer thickness [20], degree of polyelectrolyte layer interpenetration [2], surface roughness (RMS) [21], film wettability [13], and the formation/size of pores [14]. This sort of control may enable new technological opportunities, such as incorporating covalently-linked dyes as light-harvesting antennae components in graded redox potential systems for charge separation in solar energy conversion [22].…”