perovskites have received significant attention as eminent solar cell absorbers mainly due to their structural tunability and high stability compared to their three-dimensional (3D) counterparts. These materials have been mostly prepared using a solution process; thus, their optoelectronic properties and intrinsic stability depend critically on the characteristics of the precursor solutions. Here, for the first time, a double-step homogeneous precursor mixing process is employed to enhance the quality and stability of 2D perovskite precursor solutions and the corresponding solar cells. To attain the precursor solutions via such a process, a solution comprising PbI 2 and a bulky spacer such as propane-1,3-diammonium iodide (PDAI 2 ) was first made followed by the addition of a methylammonium iodide (MAI) solution. This facile double-step process enabled the formation of particularly stable solvation complexes as a result of the significantly enhanced interactions between the solvent and precursors. Hence, the obtained precursor solutions had high uniformity and stability. Additionally, the prepared 2D (PDA)(MA) n−1 Pb n I 3n+1 (n = 5, 10, and 15) films exhibited several remarkable properties, including an exceptionally smooth surface with enhanced crystallinity, highly stable crystal structures, and excellent moisture resistance, compared to the films prepared via the conventional precursor mixing process. Consequently, unsealed solar cells based on these 2D perovskite absorbers delivered power conversion efficiencies (PCEs) of up to 6.35% at n = 15 with high stability in humid air (relative humidity exceeding 70%), whereas the devices fabricated via the conventional process demonstrated considerably lower PCEs and stability. These results revealed that this newly developed process opens up new avenues in the fabrication of highly stable and efficient 2D perovskite solar cells.

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Pectin has been increasingly accepted for pharmaceutical applications including targeted drug delivery due to its biodegradability. For this purpose, the physicochemical characterization of pectin is important for fabrication of dosage forms. The present study focused on the consequence of pH on the properties of pectin. Citrus pectin, including high methoxyl pectin (CU 201, CU 401) and low methoxyl pectin (CU 701) were prepared at different pH and comparatively evaluated for their solution and solid-state properties. The results indicated that rheological behavior was depended on pH and grade of pectin. As increasing pH from 2.9 to 4.0, the viscosity of pectin solutions was significantly decreased. However, the low methoxyl pectin was less sensitive to the increment of pH from 5.0 to 7.0. Regardless of pH, the high methoxyl pectin showed shear thinning behavior while the low methoxyl pectin demonstrated Newtonian flow except for pH 2.9. The results suggested the pH dependent structural change of citrus pectin, especially for low methoxyl pectin, which was later confirmed by the results from powder X-ray diffractometry. The knowledge obtained from this research provides the basic knowledge for the specific selection of pH and grade of pectin in fabrication of drug delivery systems. Keywords: Pectin; drug delivery; pH; rheology IntroductionPectin is a natural biopolymer that has been widely applied in the food and pharmaceutical industry due to its attractive properties including gelling property and ability to modify drug release. It commonly occurs as the major part of cell wall of higher plant; however, the commercial source is still limited to citrus fruits and apple since the desired properties is controlled by the structure and chemistry of pectin from selected sources. Degree of esterification, molecular weight is the major factor that affects physicochemical properties of pectin. With regard to degree of esterification, pectin is categorized as high methoxyl pectin (HMP) and low methoxyl pectin (LMP) with degree of methoxylation of >50% and <50%, respectively. (Sriamornsak, 2003) These two groups of pectin demonstrate different gelling and other related properties which depend on various factors including amount of soluble solid (e.g. sugar content), amount of cation species and pH. HMP could form gel at higher solid content (generally more than 55%) as compared with LMP. (Yapo & Koffi, 2013) However, LMP requires