Scents are volatile compounds highly employed in a wide range of manufactured items, such as fine perfumery, household products, and functional foods. One of the main directions of the research in this area aims to enhance the longevity of scents by designing efficient delivery systems to control the release rate of these volatile molecules and also increase their stability. Several approaches to release scents in a controlled manner have been developed in recent years. Thus, different controlled release systems have been prepared, including polymers, metal–organic frameworks and mechanically interlocked systems, among others.
1. Introduction
Scents are essential components in a wide range of products which are consumed on a daily basis [1]. Thus, consumers can enjoy fragrances in a broad pool of areas, from the pleasant aroma of a fine perfume to the smell of detergents and softeners employed for washing and caring of clothes, but also many other areas, such as the food industry and body care products. This broad representation of aromas in several key industries illustrates the relevance and multivalence of the perfume industry itself [2][3]. Although in the beginning of perfumery only fragrances from natural sources were used, less than 5% of perfume ingredients come from natural sources nowadays [1]. The introduction of synthetic fragrance ingredients marked the starting point of the modern perfume industry, devoting efforts in two main directions: (i) synthesis of the ingredients originally obtained from natural sources, also known as natural identical ingredients; (ii) chemical modification of the natural identical ingredients to obtain a pool of derivatives having an analogous structure but with enhanced properties. The use of both natural and synthetic scents in fine perfumery, flavors, body care, and several household products, among other items, has an inherent handicap due to an intrinsic property of these types of compounds. Scents are characterized for their low molecular weights which allow their efficient evaporation [4][5][6]. Although the volatility is a requirement in order to enjoy the pleasant aroma of these substances, this property could also be a problem, shortening the persistence of the odor. Thus, the useful life of the product can also be reduced during its storage, not only from the starting date of consumption. With the aim of increasing the longevity of the fragrances, numerous scientists have dedicated their efforts in the development of selective and effective release systems, also known as profragrances, which allow the controlled and slow release of extremely volatile scents [7][8][9][10]. Thus, through these systems, the degradation or loss of fragrance is minimized during the process of production and storage, and also the life of use of the product by the consumer is improved.
In this scenario, the development of systems that respond to external stimuli, which has been widely used to build smart materials
[11][12][13][14][15] and to control the release of drugs
[16][17][18][19][20], among other applications, has turned out to be a suitable strategy in the research focused on scents. These systems have been effectively employed to accomplish a slow release of the fragrance. Different approaches have been developed, including reversible encapsulation, supramolecular interactions, and cleavable covalent bonds
[8]. These systems allow the release of scents in response to different stimuli. It is especially important that these stimuli are attributable to conditions which occur spontaneously in everyday use conditions, and also allow the adequate conservation of the organoleptic properties until the consumer starts using the product.
In terms of the adequate applicability, these systems must fulfil a series of requirements: (i) the design of the controlled release system should allow the sustained release according to the specific application; (ii) the release of the scent from the system should be easily controllable without causing damages to the fragrance; (iii) the system should be biocompatible.
2. Polymer- and Gel-Based Release Systems
Natural and synthetic polymers provide high versatility in order to release scents in a controlled manner
[21][22][23]. On the one hand, natural polymers usually have high affinity with several scents, beneficiating their stabilization within the polymeric matrix. On the other hand, synthetic polymers lead to a tailorable design, affording different functionalization which is incorporated in the polymeric scaffolds to obtain enhanced release operation.
Low molecular weight gels, which are obtained from a matrix of self-assembled small molecules by the establishment of non-covalent interactions, construct anisotropic structures able of immobilizing different molecules within the supramolecular network
[34][35][36][37][38], thus being useful in order to accomplish sustained release of scents.
3. Ionic Liquid-Based Release Systems
Ionic liquids are solvents consisting of bulky and unsymmetrically substituted organic cations and anions which are in the liquid phase at temperatures below 100 °C
[46]. Ionic liquids have been employed extensively in biomedical applications and as electrolytes, but also in controlled release systems and as excipients to solubilize a wide range of small molecules in water
[47][48][49][50][51][52][53][54][55]. Ionic liquids interact with the solute and disrupt the interactions of the solute with a co-solvent, integrating the molecule into a space provided by this type of molecular system. Regarding fragrance release applications, ionic liquids usually have negligible vapor pressure at ambient conditions which would potentially render excellent profragrance scaffolds.
4. Metal-Organic Framework-Based Release Systems
Metal–organic frameworks (MOFs) are a type of porous material which have several advantages over conventional microporous materials, such as the great versatility of their design and being able to easily control the pore size. MOFs are constituted of organic ligands coordinated to metal ions, forming a uniform crystalline lattice, which can be repeated in any of the three dimensions. The chemistry of MOFs has experienced great progress in the last decades due to the possibility of generating almost infinite structures from numerous commercially available metal precursors and an immense range of organic ligands, both commercial and synthesized in the laboratory. Thus, an exponential increase in the research interest in these systems has arisen. The design of the morphology of the crystalline matrix, as well as the fine-tuning of the pore size, has resulted in several applications, such as gas adsorption, heterogeneous catalysis, and selective recognition of a wide range of target molecules
[57][58][59][60][61][62][63][64][65][66].
The versatility of these metal–organic porous materials has led to the design of systems that selectively incorporate target molecules within the pores of the crystals and allow the controlled release of the cargo upon the application of different stimuli, such as light, pH, or redox. The switchable delivery is favored by incorporating stimuli-responsive organic struts within the crystalline arrays. Thus, the release of a wide variety of compounds has been attempted by using MOFs as adjustable scaffolds
[67][68][69][70][71][72][73][74][75][76][77].
Although the use of other microporous materials operating as scent nanodispenser-based systems such as zeolites has been reported
[78][79], MOFs have turned out to be fine-tunable microporous vehicles which allow an enhanced and tailorable control of the release rate of scents.
5. Organic Salt-Based Release Systems
The water-triggered release systems offer the advantage that the moisture of the air can act as a stimulus in order to achieve a slow release of the target scents. Thus, the hydrolysis of profragrances is a suitable strategy to design sustained release systems. In this scenario, hydrolyzable organic salts, which incorporate the scent precursor in their structure, have become suitable candidates to be used as profragrances.
Surfactants are a type of organic compounds having, in the same molecule, both hydrophobic and hydrophilic scaffolds, providing an amphiphilic nature which can form supramolecular associations. This inherent characteristic has led to several applications, such as detergency, food processing, and cosmetics formulation
[83][84][85][86]. Surfactants also have potential applications in the perfume industry, increasing the aroma perception time by providing a decrease in the volatility of the aroma.
6. Coumarin-Based Release Systems
Coumarin derivatives are organic chemical compounds of the benzopyrazone family which have been widely employed as photoactive systems for the delivery of drugs in a controlled manner
[88][89][90][91][92]. This photo-triggered release of prodrugs has led to envision the use of coumarin-based compounds as profragrances.
7. Cyclodextrin-Based Release Systems
Cyclodextrins are a type of truncated cone-like macrocyclic compounds constituted by oligosaccharides connected through α-1,4-glucosidic bonds. CDs have been widely employed in host–guest chemistry, encapsulating target molecules within their cavities. Thus, a rational design of different systems containing CDs, such as interlocked molecules, cross-linked hydrogels, and supramolecular polymers, have led to enhanced applications in the research field of supramolecular chemistry. The possibility of use of CD complexes as controlled release systems has allowed the application of these supramolecular systems in several biocompatible applications, such as functional foods and drug delivery
[95][96][97][98][99][100][101]. Thus, in addition to the previously mentioned γ-CD-MOF
[102], cyclodextrin macrocycles have been directly employed in the sustained release of scents. Indeed, cyclodextrins have been extensively employed in the preparation of functional food
[103], paving the way for their application in the encapsulation and release of flavor and fragrances so that the consumer perceives the unaltered organoleptic properties of the product.
8. Rotaxane-Based Release Systems
Rotaxanes are a type of mechanically interlocked molecules (MIMs) constituted by at least two components: a cyclic counterpart, also known as macrocycle or wheel, surrounding a linear counterpart, also known as axle or thread. These components are not covalently bonded, but rather are stabilized by noncovalent forces, forming a mechanical bond. The dissociation of the different counterparts is prevented by bulky groups at both ends of the thread, also known as stoppers
[113][114][115]. The research focused on rotaxanes has attracted the interest of many scientists in the area of supramolecular chemistry due to the new properties conferred by the mechanical bond and also for the dynamics of the counterparts and the possibility of exercising control over this motion. Thus, the chemistry of rotaxanes has resulted in several applications in the area of molecular machinery
[116][117][118][119][120][121][122][123][124][125]. A rational design of the rotaxanated architectures has led to the development of biological applications, highlighting the switchable release of anticancer drugs
[126][127][128][129][130]. The versatility of the structural design of these intertwined compounds has also led to envision their use as effective profragrances.